feat: add agent configuration support to init command

[arittr]

• Add --agent flag to commitment init for setting default agent
• Implement manual argv parsing to handle Commander.js subcommand option conflict
• Display configured agent in init success message
• Update README with agent configuration examples for hook setup

🤖 Generated with Claude via commitment

[arittr]

This change is part of the following stack:

• refactor(agents): extract commit message cleaning logic to shared utilities #58
  • feat: add agent factory with ts-pattern for type-safe instantiation #59
    • feat: implement Gemini agent with CLI integration #60
      • feat: add Gemini agent support to evaluation system #61
        • refactor: establish SUPPORTED_AGENTS as single source of truth #62
          • docs: update documentation to reflect Gemini agent support #63
            • feat: add agent configuration support to init command #64 ◀
              • fix: preserve staged changes after pre-commit hook #65

_{Change managed by git-spice.}