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_kernel.go

@@ -54,9 +54,10 @@ Subcommands:
 func (d *deps) newKernelPullCommand() *cobra.Command {
 	var force bool
 	cmd := &cobra.Command{
-		Use:   "pull <name>",
-		Short: "Download a cataloged kernel bundle",
-		Args:  exactArgsUsage(1, "usage: banger kernel pull <name> [--force]"),
+		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 {