feat: Shell completion generation

[boneskull]

Summary

Add support for generating shell completion scripts (bash, zsh, fish) similar to yargs' .completion() feature.

Use Case

When migrating from yargs to bargs, shell completion is one of the features that doesn't have a direct equivalent. Power users who rely on tab completion for CLI tools would benefit from this.

Proposed API

Something like:

const cli = bargs('mytool')
  .command('build', buildParser, buildHandler)
  .command('test', testParser, testHandler);

// Generate completion script
cli.completionScript('bash'); // or 'zsh', 'fish'

Or perhaps a built-in completion command that outputs the appropriate script:

mytool completion bash >> ~/.bashrc
mytool completion zsh >> ~/.zshrc

Priority

Low - This is a nice-to-have for power users, not a blocker for CLI functionality.

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Feature request from migrating modestbench from yargs to bargs 🎸