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>

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{
@@ -175,8 +155,9 @@ func (d *deps) newImagePullCommand() *cobra.Command {
 		sizeRaw string
 	)
 	cmd := &cobra.Command{
-		Use:   "pull <name-or-oci-ref>",
-		Short: "Pull an image bundle (catalog name) or OCI image and register it",
+		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
-subcommand lands).
+Use 'banger image list' to see installed images.
 `),
 		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
 }