feat: add structured decision format and auto-close for rejected PRs

Enhance the Claude PR review workflow with:
- Remove redundant criteria mention (already in CONTRIBUTING.md)
- Add file access restrictions to prevent unnecessary file reads
- Require structured DECISION output format for automation
- Auto-close rejected PRs with explanatory comment

🤖 Generated with Claude Code (https://claude.ai/code)
Co-Authored-By: Claude <[email protected]>