- Fix MENTIONED_RE/CLOSED_BY_RE to match real GitLab format
('mentioned in issue #N' / 'mentioned in merge request !N')
- Add GITLAB_URL_RE + parse_url_refs() for full URL extraction
- Add extract_refs_from_descriptions() -> source_method='description_parse'
- Add extract_refs_from_user_notes() -> source_method='note_parse'
- Wire both into orchestrator after system note extraction
- 36 tests: regex fix, URL parsing, integration, idempotency