Add PR Testing Agent documentation

Creates a specialized agent definition for comprehensive PR testing validation that is skeptical of testing claims unless verified, assumes automated tests have gaps, and requires clear communication about what was tested vs what needs manual testing.

Key sections:
- Agent behavior guidelines (skeptical stance, clear communication)
- Testing requirements by change type (Ruby, JS/TS, builds, webpack, CI, generators, engines)
- Environment-specific limitations (Conductor workspace, Ruby 3.4 SSL issues)
- Pre-merge testing checklist template
- Real-world scenario examples with good/bad examples

This guide pulls all testing information from existing documentation (testing-build-scripts.md, SWITCHING_CI_CONFIGS.md, TESTING_LOCALLY.md, CLAUDE.md) and integrates project-specific tools (bin/ci-rerun-failures, bin/ci-switch-config, etc.).

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

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

Author: justin808