feat: shell completion (bash/zsh/fish)

[nmhjklnm]

Summary

Add tab completion for cac commands.

Proposed API

cac completion bash > /etc/bash_completion.d/cac
cac completion zsh > ~/.zfunc/_cac
cac completion fish > ~/.config/fish/completions/cac.fish

What to complete

• Top-level: env, claude, self, docker, ls, check, -v, help
• cac env: create, set, ls, rm, check, clone, import, export
• cac claude: install, ls, pin, uninstall
• cac self: update, delete
• cac <name>: complete from existing environment names
• cac env set <name>: complete env names, then keys (proxy, version)
• cac claude pin <ver>: complete from installed versions

Implementation

• Add src/cmd_completion.sh
• Register in main.sh dispatcher