Add CI validation script for checking unauthorized expose_secret() call
sites. The script validates that all uses of expose_secret() are in
approved locations (SecretFingerprint and test code).
Also add verification note summarizing the bead completion status.
Per pdftract-5l9m acceptance criteria:
- CI grep guard rejects unauthorized expose_secret() call sites
- Verification documents existing SecretString wrapping status
Co-Authored-By: Claude Code <noreply@anthropic.com>
