Add PR Testing Agent documentation (#2109)

### Summary

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

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.).

### Key Features

- **Skeptical stance**: Assumes "tests pass" needs verification
- **Testing by change type**: Ruby code, JS/TS, builds, webpack, CI,
generators, Rails engines
- **Environment awareness**: Documents Conductor workspace limitations
clearly
- **Pre-merge checklist**: Comprehensive checklist template based on
files changed
- **Communication templates**: PR comment and commit message examples
- **Real-world scenarios**: Good vs bad examples for common testing
claims

### Pull Request checklist

- [x] Add/update test to cover these changes - N/A, documentation only
- [x] Update documentation - Added new agent documentation
- [x] Update CHANGELOG file - N/A, documentation only

<!-- This is an auto-generated comment: release notes by coderabbit.ai
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## Summary by CodeRabbit

* **Documentation**
* Added comprehensive PR Testing Agent documentation with testing
requirements by change type, verification strategies, pre-merge testing
checklists, and best practices for ensuring quality before integration.

<sub>✏️ Tip: You can customize this high-level summary in your review
settings.</sub>

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

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Co-authored-by: Claude <[email protected]>

Author: justin808