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.
