Add shell completion generation (bash/zsh/fish)

[lonexreb]

Summary

Auto-generate shell completions for all CLI commands including dynamically discovered site commands.

Details

Typer has built-in completion support via --install-completion and --show-completion. However, dynamically generated site commands (e.g., site2cli run example.com <action>) need custom completion logic.

Implementation

• Enable Typer's built-in completions for static commands
• Add dynamic completion for site names (from registry) and action names
• site2cli --install-completion for one-time setup
• Document in README

Files

• src/site2cli/cli.py — Typer app completion callbacks