Split PR Testing documentation into Agent + Guide

Splits the 1,172-line combined file into two focused documents:

**pr-testing-agent.md** (493 lines) - The "what" and "why":
- Agent behavior and principles (skeptical stance, communication style)
- Testing requirements by change type (8 sections: Ruby, JS/TS, Build, Webpack, CI, Generators, Playwright E2E, Rails Engine)
- Environment-specific testing limitations
- Success criteria for well-tested PRs
- Agent mindset summary

**pr-testing-guide.md** (693 lines) - The "how":
- When to use and how to invoke
- Using with Claude Code (complete workflow examples)
- Integration with existing workflows
- Pre-merge testing checklist
- Communication templates (PR comments, commit messages)
- Real-world testing scenarios (good vs bad examples)
- Integration with project tools

Benefits:
- Each file has a clear, focused purpose
- Easier to reference specific parts
- Better for both human reading and AI assistant usage
- Agent file stays focused on behavior/requirements
- Guide file focuses on practical usage

Cross-references updated between files for easy navigation.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: justin808