feat: shipnode completion — shell completion for bash/zsh/fish

[devalade]

What to build

Add a shipnode completion <bash|zsh|fish> command that emits a shell-completion script to stdout, following the standard pattern (e.g. kubectl completion zsh, gh completion -s zsh).

Completion should cover:

• Subcommands and their subsubcommands (env pull, backup list, user remove, config show, etc.)
• All flags, with descriptions where helpful (--dry-run, --follow, --source, --file, --lines, --steps, etc.)
• Dynamic values where cheap to compute:
  • rollback --to <release> → list release timestamps from remotePath/releases/
  • user remove <name> → list non-system users (mirrors user list)
  • --source for logs → pm2|caddy|system|all
  • --preset for init (once feat: shipnode completion — shell completion for bash/zsh/fish #11 lands) → list of supported preset names

Static completion (subcommands + flags) is enough to ship v1; dynamic completions can land as a follow-up.

Acceptance criteria

• shipnode completion bash | source enables completion in the current bash session
• shipnode completion zsh > \"${fpath[1]}/_shipnode\" works, then compinit picks it up
• shipnode completion fish | source works in fish
• Tab-completing shipnode <TAB> lists all top-level subcommands
• Tab-completing shipnode env <TAB> lists diff, pull, plus the flags for push
• Tab-completing shipnode logs --source <TAB> (v1 dynamic) lists pm2 caddy system all
• README section: one-liner install for each shell

Blocked by

None - can start immediately (dynamic completions for --preset depend on #11)