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/gitlab/client.rs

@@ -627,6 +627,15 @@ impl GitLabClient {
         self.fetch_all_pages(&path).await
     }
 
+    pub async fn fetch_issue_links(
+        &self,
+        gitlab_project_id: i64,
+        issue_iid: i64,
+    ) -> Result<Vec<crate::gitlab::types::GitLabIssueLink>> {
+        let path = format!("/api/v4/projects/{gitlab_project_id}/issues/{issue_iid}/links");
+        coalesce_not_found(self.fetch_all_pages(&path).await)
+    }
+
     pub async fn fetch_mr_diffs(
         &self,
         gitlab_project_id: i64,