API endpoints:
- GET/POST/DELETE /api/v1/orgs/{org}/custom-fields — org field definitions
- GET/PUT /api/v1/repos/{owner}/{repo}/metadata — repo-scoped field values
- GET/PUT /api/v1/repos/{owner}/{repo}/issues/{index}/custom-fields — issue values
All endpoints use field names as keys (not IDs) for ergonomic access.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>