Add shell completion via `bonsai completion [bash|zsh|fish]`

[LastStep]

Background

Bonsai uses Cobra, which ships a GenBashCompletionFile / GenZshCompletion / GenFishCompletion API. Most Cobra CLIs expose a completion subcommand so users can wire completions into their shells (bonsai completion zsh > ~/.zfunc/_bonsai, etc.).

Bonsai currently has no completion support — every command, flag, and installed-ability name is a tab-dead-end.

Acceptance criteria

• New subcommand bonsai completion [bash|zsh|fish|powershell] added to cmd/.
• Running it prints the shell-appropriate completion script to stdout.
• Standard Cobra DisableDefaultCmd: false / explicit CompletionOptions wiring.
• README "Installation" section gets a short "Shell completion" subsection with per-shell install snippets (bash/zsh/fish).
• make build + existing tests pass.

Hints

• See Cobra's completion docs.
• Drop the file at cmd/completion.go; register in cmd/root.go.
• Completions for bonsai add <skill> ability-name arguments would be a nice stretch but are out-of-scope for this issue — file a follow-up.

Why this is a good first issue

Self-contained new command, no schema changes, no interaction with the TUI harness. Visible to every user.