Implement shell completions with clap_complete
- Add clap_complete and home dependencies
- Add CompletionShell enum (bash, zsh, fish, PowerShell, elvish)
- Create completions module with generate_completion() function
- Add completions subcommand to CLI with --print flag
- Support both stdout output and file installation
- Auto-detect shell-specific completion directories
- Provide shell-specific installation instructions
- Completions work for all commands, flags, and value enums
Resolves exs-460