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/cli/commands/mod.rs

@@ -3,6 +3,7 @@ pub mod count;
 pub mod doctor;
 pub mod drift;
 pub mod embed;
+pub mod explain;
 pub mod file_history;
 pub mod generate_docs;
 pub mod ingest;
@@ -29,6 +30,7 @@ pub use count::{
 pub use doctor::{DoctorChecks, print_doctor_results, run_doctor};
 pub use drift::{DriftResponse, print_drift_human, print_drift_json, run_drift};
 pub use embed::{print_embed, print_embed_json, run_embed};
+pub use explain::{ExplainResponse, print_explain_human, print_explain_json, run_explain};
 pub use file_history::{print_file_history, print_file_history_json, run_file_history};
 pub use generate_docs::{print_generate_docs, print_generate_docs_json, run_generate_docs};
 pub use ingest::{