cli: rewrite help text for AI-driven discovery

Frontier models tend to discover a CLI by running --help, scanning
the Long description, and inferring the dominant workflow from the
examples. Today's banger help reads like a man page index — every
verb has a one-line Short and nothing else. This rewrites the
groups (banger, vm, vm workspace, image, kernel, system,
ssh-config) so each landing page answers "what is this for, what's
the 80% command, what comes next" in three to ten lines, with
runnable examples.

Also disambiguates the near-twin lifecycle commands so a model
reading the subcommand index can tell stop/kill/delete apart at a
glance:

  start    Start a stopped VM
  stop     Stop a running VM gracefully
  restart  Stop then start a VM
  kill     Force-kill a VM (use when 'vm stop' hangs)
  delete   Stop a VM and remove its disks (irreversible)

vm create / vm ssh / vm logs / vm show pick up Long descriptions
and examples for the same reason. No behaviour changes; help text
only.

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

internal/cli/commands_image.go

@@ -15,8 +15,28 @@ import (
 func (d *deps) newImageCommand() *cobra.Command {
 	cmd := &cobra.Command{
 		Use:   "image",
-		Short: "Manage images",
-		RunE:  helpNoArgs,
+		Short: "Pull and manage banger images (rootfs + kernel + work-seed)",
+		Long: strings.TrimSpace(`
+A banger image bundles a rootfs.ext4, a kernel, an optional initrd
++ modules, and an optional work-seed (the snapshot used to populate
+each new VM's /root). Most users only need 'banger image pull
+<catalog-name>' for the cataloged paths (see internal/imagecat),
+or 'banger image pull <oci-ref>' for an OCI image.
+
+Subcommands:
+  pull       fetch a bundle by catalog name OR pull an OCI image
+  register   point banger at an existing local rootfs (advanced)
+  promote    copy a registered image's files into banger's managed dir
+  list       show what's installed
+  show       print one image's full record as JSON
+  delete     remove an image (no VMs may reference it)
+`),
+		Example: strings.TrimSpace(`
+  banger image pull debian-bookworm
+  banger image pull docker.io/library/alpine:3.20 --kernel-ref generic-6.12
+  banger image list
+`),
+		RunE: helpNoArgs,
 	}
 	cmd.AddCommand(
 		d.newImageRegisterCommand(),