cli: shell completion via cobra + dynamic resource name lookups

Re-enable cobra's default `completion` subcommand (`banger completion
bash|zsh|fish|powershell`). Plus live resource-name suggestions that
hit the running daemon via the same RPC the real commands use:

  vm start/stop/restart/delete/kill/set         → completeVMNames (variadic)
  vm ssh/show/logs/stats/ports/...              → completeVMNameOnlyAtPos0
  vm session list/start                         → completeVMNameOnlyAtPos0
  vm session show/logs/stop/kill/attach/send    → completeSessionNames (vm + session)
  image show/delete/promote                     → completeImageNameOnlyAtPos0
  kernel show/rm                                → completeKernelNameOnlyAtPos0
  vm run/create --image, image pull/register --kernel-ref → flag-value completion

Design notes in internal/cli/completion.go: completers never auto-start
the daemon (ping-check, bail with NoFileComp on miss), so tab-completion
stays a zero-cost probe. Variadic completers exclude already-entered
args to avoid duplicate suggestions.

README: install recipes for bash / zsh / fish.

README.md

@@ -32,6 +32,28 @@ Installs `banger` (CLI), `bangerd` (daemon, auto-starts on first
 CLI call), and `banger-vsock-agent` (companion, under
 `$PREFIX/lib/banger/`).
 
+### Shell completion
+
+`banger` ships completion scripts for bash, zsh, fish, and
+powershell. Tab-completion covers subcommands, flags, and live
+resource names (VM, image, kernel, session) looked up from the
+daemon. With the daemon down, resource completion silently
+returns nothing — no file-completion fallback.
+
+```bash
+# bash (system-wide)
+banger completion bash | sudo tee /etc/bash_completion.d/banger
+
+# zsh (user-local; ~/.zfunc must be on fpath)
+banger completion zsh > ~/.zfunc/_banger
+
+# fish
+banger completion fish > ~/.config/fish/completions/banger.fish
+```
+
+`banger completion --help` shows the shell-specific loading
+recipes.
+
 ## `vm run`
 
 One command, four common shapes: