Add CONTRIBUTING.md with contribution guidelines

[supmo668]

Problem

The repo has no CONTRIBUTING.md, making it unclear for contributors how to:

• Set up the project locally
• Follow commit conventions
• Structure new commands, agents, or templates
• Test changes
• Submit PRs

Proposed Content

A CONTRIBUTING.md covering:

1. Getting Started

• Clone & setup instructions
• How to copy commands into your own project vs. developing on this repo

2. Command Authoring Guide

• Required YAML frontmatter (description, argument-hint)
• Phased structure with checkpoints
• GATE pattern for user interaction points
• Agent dispatch patterns (parallel dual-agent, sequential triple, multi-agent review)
• File output conventions (.claude/PRPs/{category}/)
• Naming: kebab-case, -batch suffix for batch variants

3. Agent Authoring Guide

• Agent file location (.claude/agents/)
• Required structure (role, constraints, output format)
• Model specification (sonnet default)
• Dispatch via Task tool with subagent_type

4. Commit & PR Conventions

• Conventional commits (feat:, fix:, docs:, etc.)
• Branch naming (feat/, fix/, docs/)
• PR template adherence
• Base branch: development

5. Testing

• Since these are prompt files, testing = running the command in Claude Code and verifying output
• Checklist for command validation (frontmatter parses, phases execute, gates pause, artifacts generate to correct paths)

6. Code of Conduct

• Standard Contributor Covenant or similar

Additional Context

Related PR: #22 (batch commands + README consolidation) — an example of the kind of contribution this guide would support.

This could also live partially in a GitHub Discussion if the maintainer prefers async community input on the guidelines.