Add CI Pipeline Gates for Docs Doctrine Compliance

Add CI gates that block non-compliant commits, per docs-doctrine §10.3–10.4.

This is the enforcement layer that makes the doctrine non-aspirational. Without CI checks, rules will drift.

Acceptance Criteria

• [ ] Add GitHub Actions workflow (.github/workflows/docs-compliance.yml)
• [ ] Gate 1: Run scripts/validate-docs — fail build on any violation
• [ ] Gate 2: Regenerate INDEX.md and diff — fail if stale
• [ ] Gate 3: Run sync-claude and diff — fail if derived CLAUDE.md is stale
• [ ] Gate 4: Detect duplicate canonicals via hash comparison
• [ ] Gate 5: Timestamp SLA validation (derived files not older than SLA allows)
• [ ] Block merge on any gate failure
• [ ] Add status badge to repo README

Source

docs/governance/docs-doctrine.md — Migration Phase 5 ("Enforce") + §10.3–10.5