Add shell completion command

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Task summary

Add a \commit-echo completion\ subcommand that generates shell completion scripts for bash, zsh, and fish shells.

Why this is a good first issue

Small, self-contained addition to the CLI entry point. Teaches Commander.js API basics (subcommand registration, action handling) and has an immediate payoff for users who want tab-completion in their terminal.

Suggested files or areas

• \src/index.ts\ — Register the new \completion\ subcommand, use Commander's built-in .createCompletion()\ or output completion script

Acceptance criteria

• \commit-echo completion\ prints a help message showing available shells
• \commit-echo completion bash\ outputs a bash completion script
• \commit-echo completion zsh\ outputs a zsh completion script
• \commit-echo completion fish\ outputs a fish completion script
• 
  pm run build\ passes with no errors

Extra context

Commander v13 has built-in completion support via .createCompletion(). See the Commander docs for reference. The existing \src/index.ts\ patterns (e.g., .command('history')) can be followed.