Shell completions (bash/zsh/fish)

[Chessing234]

Generate shell completions for all CLI subcommands and flags so users get tab-completion for chirp add, chirp daemon status, --list, --json, etc.

Suggested approach:

• Use clap with clap_complete to auto-generate completions, or hand-write them
• Add chirp completions <shell> subcommand that prints the completion script
• Document installation in README: chirp completions zsh > ~/.zfunc/_chirp