- Add TypeRig IssueType constant for rig identity beads
- Add TypeRig to IsValid() switch statement
- Add IsBuiltIn() method for multi-repo hydration trust checks
- Add Crystallizes field to Issue struct and ComputeContentHash
Fixes validation rejecting documented issue types like 'rig' and 'agent'.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>