cli,docs: trivial polish for v0.1.0

A pre-release audit collected ~12 trivial-effort UX and code-hygiene
items. Rolling them up here so the v0.1.0 commit log isn't littered
with one-line tweaks.

CLI help / completion:
  * commands_image.go: drop dangling reference to a `banger image
    catalog` subcommand that doesn't exist; replace with a pointer
    to `banger image list`.
  * commands_image.go: --size flag example was "4GiB" but the parser
    rejects that suffix. Change example to "4G". (Parser-side fix
    is in a separate concern.)
  * commands_image.go + completion.go: image pull now wires a
    catalog completer (falls back to local image names since there's
    no image-catalog RPC yet); image show / delete / promote already
    completed local names.
  * commands_kernel.go + completion.go: kernel pull now wires a new
    completeKernelCatalogNameOnlyAtPos0 backed by the kernel.catalog
    RPC, so tab-complete suggests pullable kernels.
  * commands_vm.go: vm stats and vm set now have Long + Example
    blocks (peers all do); --from flag description updated to spell
    out the relationship to --branch.

README:
  * Define "golden image" inline at first use.
  * Add a one-line Requirements block above Quick Start so users
    hit the firecracker / KVM dependency before `make build`.

Code hygiene:
  * dashIfEmpty / emptyDash were the same function. Deleted
    emptyDash, retargeted three call sites.
  * formatBytes (introduced today in image cache prune) duplicated
    humanSize. Consolidated to humanSize, now with a space ("1.2
    GiB" not "1.2GiB"). formatters_test.go expectations updated.

Logging chattiness:
  * "operation started" (logger.go), "daemon request canceled"
    (daemon.go), and "helper rpc completed" (roothelper.go) all
    fired at INFO per RPC. Downgraded to DEBUG so routine shell
    completions don't spam syslog.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

README.md

@@ -2,6 +2,8 @@
 
 One-command development sandboxes on Firecracker microVMs.
 
+**Requirements:** Linux + KVM (`/dev/kvm`), `firecracker` on PATH (or `firecracker_bin` in config).
+
 ## Quick start
 
 ```bash
@@ -10,10 +12,10 @@ sudo ./build/bin/banger system install --owner "$USER"
 banger vm run --name sandbox
 ```
 
-That's it. `banger vm run` auto-pulls the default golden image (Debian
+That's it. `banger vm run` auto-pulls the default golden image (a pre-built
-bookworm with systemd, sshd, Docker CE, git, jq, mise, and the usual
+Debian rootfs with sshd, mise, and the usual dev tools: Debian bookworm with
-dev tools) and kernel, creates a VM, starts it, and drops you into
+systemd, sshd, Docker CE, git, jq, and mise) and kernel, creates a VM, starts
-an interactive ssh session. First run takes a couple minutes (bundle
+it, and drops you into an interactive ssh session. First run takes a couple minutes (bundle
 download); subsequent `vm run`s are seconds.
 
 ## Supported host path

internal/cli/commands_image.go

@@ -106,7 +106,7 @@ in flight when you run prune, that pull may fail and need a retry.
 				verb = "would free"
 			}
 			_, err = fmt.Fprintf(out, "%s %s across %d blob(s) in %s\n",
-				verb, formatBytes(result.BytesFreed), result.BlobsFreed, result.CacheDir)
+				verb, humanSize(result.BytesFreed), result.BlobsFreed, result.CacheDir)
 			return err
 		},
 	}
@@ -114,26 +114,6 @@ in flight when you run prune, that pull may fail and need a retry.
 	return cmd
 }
 
-// formatBytes renders a byte count as a short human-readable string
-// (e.g. "1.2 GiB", "456 MiB"). Zero stays "0 B" for clarity.
-func formatBytes(n int64) string {
-	const (
-		ki = 1024
-		mi = ki * 1024
-		gi = mi * 1024
-	)
-	switch {
-	case n >= gi:
-		return fmt.Sprintf("%.1f GiB", float64(n)/float64(gi))
-	case n >= mi:
-		return fmt.Sprintf("%.1f MiB", float64(n)/float64(mi))
-	case n >= ki:
-		return fmt.Sprintf("%.1f KiB", float64(n)/float64(ki))
-	default:
-		return fmt.Sprintf("%d B", n)
-	}
-}
-
 func (d *deps) newImageRegisterCommand() *cobra.Command {
 	var params api.ImageRegisterParams
 	cmd := &cobra.Command{
@@ -177,6 +157,7 @@ func (d *deps) newImagePullCommand() *cobra.Command {
 	cmd := &cobra.Command{
 		Use:               "pull <name-or-oci-ref>",
 		Short:             "Pull an image bundle (catalog name) or OCI image and register it",
+		ValidArgsFunction: d.completeImageCatalogNameOnlyAtPos0,
 		Long: strings.TrimSpace(`
 Pull an image into banger. Two paths:
 
@@ -190,8 +171,7 @@ Pull an image into banger. Two paths:
       banger's guest agents. --kernel-ref or direct --kernel/--initrd/
       --modules are required.
 
-Use 'banger image catalog' to see available catalog names (once that
+Use 'banger image list' to see installed images.
-subcommand lands).
 `),
 		Example: strings.TrimSpace(`
   banger image pull debian-bookworm
@@ -235,7 +215,7 @@ subcommand lands).
 	cmd.Flags().StringVar(&params.InitrdPath, "initrd", "", "initrd path")
 	cmd.Flags().StringVar(&params.ModulesDir, "modules", "", "modules dir")
 	cmd.Flags().StringVar(&params.KernelRef, "kernel-ref", "", "name of a cataloged kernel (see 'banger kernel list')")
-	cmd.Flags().StringVar(&sizeRaw, "size", "", "ext4 image size (e.g. 4GiB); defaults to content + 25%, min 1GiB")
+	cmd.Flags().StringVar(&sizeRaw, "size", "", "ext4 image size (e.g. 4G); defaults to content + 25%, min 1GiB")
 	_ = cmd.RegisterFlagCompletionFunc("kernel-ref", d.completeKernelNames)
 	return cmd
 }

internal/cli/commands_kernel.go

@@ -57,6 +57,7 @@ func (d *deps) newKernelPullCommand() *cobra.Command {
 		Use:               "pull <name>",
 		Short:             "Download a cataloged kernel bundle",
 		Args:              exactArgsUsage(1, "usage: banger kernel pull <name> [--force]"),
+		ValidArgsFunction: d.completeKernelCatalogNameOnlyAtPos0,
 		RunE: func(cmd *cobra.Command, args []string) error {
 			layout, _, err := d.ensureDaemon(cmd.Context())
 			if err != nil {

internal/cli/commands_vm.go

@@ -185,7 +185,7 @@ Three modes:
 	cmd.Flags().StringVar(&workDiskSize, "disk-size", model.FormatSizeBytes(defaults.WorkDiskSizeBytes), "work disk size")
 	cmd.Flags().BoolVar(&natEnabled, "nat", false, "enable NAT")
 	cmd.Flags().StringVar(&branchName, "branch", "", "create and switch to a new guest branch")
-	cmd.Flags().StringVar(&fromRef, "from", "HEAD", "base ref for --branch")
+	cmd.Flags().StringVar(&fromRef, "from", "HEAD", "git ref to branch from when --branch is set (default: HEAD)")
 	cmd.Flags().BoolVar(&removeOnExit, "rm", false, "delete the VM after the ssh session / command exits")
 	cmd.Flags().BoolVar(&includeUntracked, "include-untracked", false, "also copy untracked non-ignored files into the guest workspace (default: tracked files only)")
 	cmd.Flags().BoolVar(&dryRun, "dry-run", false, "list the files that would be copied into the guest workspace and exit without creating a VM")
@@ -583,6 +583,13 @@ func (d *deps) newVMSetCommand() *cobra.Command {
 	cmd := &cobra.Command{
 		Use:   "set <id-or-name>...",
 		Short: "Update stopped VM settings",
+		Long: strings.TrimSpace(`
+Reconfigure one or more stopped VMs. The VM must be stopped before
+reconfiguring — start it again with 'banger vm start' to apply the new settings.
+`),
+		Example: strings.TrimSpace(`
+  banger vm set dev --vcpu 4 --memory 8192
+`),
 		Args:              minArgsUsage(1, "usage: banger vm set [--vcpu N] [--memory MiB] [--disk-size SIZE] [--nat|--no-nat] <id-or-name>..."),
 		ValidArgsFunction: d.completeVMNames,
 		RunE: func(cmd *cobra.Command, args []string) error {
@@ -760,7 +767,7 @@ func (d *deps) newVMWorkspacePrepareCommand() *cobra.Command {
 	}
 	cmd.Flags().StringVar(&guestPath, "guest-path", "/root/repo", "guest workspace path")
 	cmd.Flags().StringVar(&branchName, "branch", "", "create and switch to a new guest branch")
-	cmd.Flags().StringVar(&fromRef, "from", "HEAD", "base ref for --branch")
+	cmd.Flags().StringVar(&fromRef, "from", "HEAD", "git ref to branch from when --branch is set (default: HEAD)")
 	cmd.Flags().StringVar(&mode, "mode", string(model.WorkspacePrepareModeShallowOverlay), "workspace mode: shallow_overlay, full_copy, metadata_only")
 	cmd.Flags().BoolVar(&includeUntracked, "include-untracked", false, "also copy untracked non-ignored files into the guest workspace (default: tracked files only)")
 	cmd.Flags().BoolVar(&dryRun, "dry-run", false, "list the files that would be copied and exit without touching the guest")
@@ -858,6 +865,15 @@ func (d *deps) newVMStatsCommand() *cobra.Command {
 	return &cobra.Command{
 		Use:   "stats <id-or-name>",
 		Short: "Show VM stats",
+		Long: strings.TrimSpace(`
+Print real-time resource statistics for a running VM as a JSON object,
+including CPU usage, memory balloon metrics, and disk I/O counters.
+Pipe into 'jq' for quick field extraction, e.g. banger vm stats dev | jq .mem.
+`),
+		Example: strings.TrimSpace(`
+  banger vm stats dev
+  banger vm stats dev | jq .
+`),
 		Args:              exactArgsUsage(1, "usage: banger vm stats <id-or-name>"),
 		ValidArgsFunction: d.completeVMNameOnlyAtPos0,
 		RunE: func(cmd *cobra.Command, args []string) error {

internal/cli/completion.go

@@ -160,3 +160,32 @@ func (d *deps) completeKernelNames(cmd *cobra.Command, args []string, toComplete
 	}
 	return filterPrefix(names, args, toComplete), cobra.ShellCompDirectiveNoFileComp
 }
+
+// completeKernelCatalogNameOnlyAtPos0 completes kernel names from the
+// remote catalog (pulled + available) at position 0 only.
+func (d *deps) completeKernelCatalogNameOnlyAtPos0(cmd *cobra.Command, args []string, toComplete string) ([]string, cobra.ShellCompDirective) {
+	if len(args) > 0 {
+		return nil, cobra.ShellCompDirectiveNoFileComp
+	}
+	socket, ok := d.daemonSocketForCompletion(cmd.Context())
+	if !ok {
+		return nil, cobra.ShellCompDirectiveNoFileComp
+	}
+	result, err := rpc.Call[api.KernelCatalogResult](cmd.Context(), socket, "kernel.catalog", api.Empty{})
+	if err != nil {
+		return nil, cobra.ShellCompDirectiveNoFileComp
+	}
+	names := make([]string, 0, len(result.Entries))
+	for _, entry := range result.Entries {
+		if entry.Name != "" {
+			names = append(names, entry.Name)
+		}
+	}
+	return filterPrefix(names, args, toComplete), cobra.ShellCompDirectiveNoFileComp
+}
+
+// completeImageCatalogNameOnlyAtPos0 falls back to the locally-installed
+// image list (there is no remote image catalog RPC today).
+func (d *deps) completeImageCatalogNameOnlyAtPos0(cmd *cobra.Command, args []string, toComplete string) ([]string, cobra.ShellCompDirective) {
+	return d.completeImageNameOnlyAtPos0(cmd, args, toComplete)
+}

internal/cli/printers.go

@@ -52,14 +52,6 @@ func dashIfEmpty(s string) string {
 	return s
 }
 
-func emptyDash(value string) string {
-	value = strings.TrimSpace(value)
-	if value == "" {
-		return "-"
-	}
-	return value
-}
-
 // -- generic printers -----------------------------------------------
 
 func printJSON(out anyWriter, v any) error {
@@ -165,9 +157,9 @@ func printVMPortsTable(out anyWriter, result api.VMPortsResult) error {
 			w,
 			"%s\t%s\t%s\t%s\n",
 			row.Proto,
-			emptyDash(row.Endpoint),
+			dashIfEmpty(row.Endpoint),
-			emptyDash(row.Process),
+			dashIfEmpty(row.Process),
-			emptyDash(row.Command),
+			dashIfEmpty(row.Command),
 		); err != nil {
 			return err
 		}

internal/daemon/daemon.go

@@ -353,7 +353,7 @@ func (d *Daemon) watchRequestDisconnect(conn net.Conn, reader *bufio.Reader, met
 			default:
 			}
 			if d.logger != nil {
-				d.logger.Info("daemon request canceled", "method", method, "remote", conn.RemoteAddr().String(), "error", err.Error())
+				d.logger.Debug("daemon request canceled", "method", method, "remote", conn.RemoteAddr().String(), "error", err.Error())
 			}
 			cancel()
 			return

internal/daemon/logger.go

@@ -66,7 +66,7 @@ func (d *Daemon) beginOperation(ctx context.Context, name string, attrs ...any)
 		allAttrs = append([]any{"op_id", opID}, allAttrs...)
 	}
 	if d.logger != nil {
-		d.logger.Info("operation started", append([]any{"operation", name}, allAttrs...)...)
+		d.logger.Debug("operation started", append([]any{"operation", name}, allAttrs...)...)
 	}
 	now := time.Now()
 	return &operationLog{

internal/roothelper/roothelper.go

@@ -395,7 +395,7 @@ func (s *Server) handleConn(conn net.Conn) {
 		if !resp.OK && resp.Error != nil {
 			s.logger.Error("helper rpc failed", "method", req.Method, "op_id", req.OpID, "duration_ms", duration, "code", resp.Error.Code, "message", resp.Error.Message)
 		} else {
-			s.logger.Info("helper rpc completed", "method", req.Method, "op_id", req.OpID, "duration_ms", duration)
+			s.logger.Debug("helper rpc completed", "method", req.Method, "op_id", req.OpID, "duration_ms", duration)
 		}
 	}
 	_ = json.NewEncoder(conn).Encode(resp)