Add end-to-end support for GitLab issue link fetching:
- New GitLabIssueLink type + fetch_issue_links API client method
- Migration 029: add issue_links job type and watermark column
- issue_links.rs: bidirectional entity_reference storage with
self-link skip, cross-project fallback, idempotent upsert
- Drain pipeline in orchestrator following mr_closes_issues pattern
- Display related issues in 'lore show issues' output
- --no-issue-links CLI flag with config, autocorrect, robot-docs
- 6 unit tests for storage logic
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