feat(sync): fetch and store GitLab issue links (bd-343o)

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

src/core/db.rs

@@ -97,6 +97,10 @@ const MIGRATIONS: &[(&str, &str)] = &[
         "028",
         include_str!("../../migrations/028_surgical_sync_runs.sql"),
     ),
+    (
+        "029",
+        include_str!("../../migrations/029_issue_links_job_type.sql"),
+    ),
 ];
 
 pub fn create_connection(db_path: &Path) -> Result<Connection> {