feat(showcase): add /eval PR comment-triggered GitHub Actions workflow

Comment-triggered CI workflow for per-PR evaluation:
- Permission gate (write collaborator only)
- /eval d5 [slug,...] comment parsing with slug validation
- Depot 16-core runner with full git history
- Posts running/result comments on the PR
- Scope detection, baseline comparison, JSON output
- 45-minute timeout with configurable parallel runners

Author: jpr5