Add a CONTRIBUTING.md to guide external and internal contributors to the CLI.

Should Cover

• Getting started / development setup
• Coding standards and conventions
• Testing expectations
• Commit message conventions (conventional commits)
• PR process and review expectations
• Pointers to DEVELOPMENT.md for deeper technical context

Notes

• AGENTS.md covers AI-specific coding guidance — CONTRIBUTING.md should reference it rather than duplicate content that applies to both human and AI contributors (e.g. code patterns, testing, commit conventions)
• Similarly, AGENTS.md should reference CONTRIBUTING.md for general contribution guidelines to avoid maintaining the same information in two places