feat(who): expand expert + overlap queries with mr_file_changes and mr_reviewers

Chain: bd-jec (config flag) -> bd-2yo (fetch MR diffs) -> bd-3qn6 (rewrite who queries)

- Add fetch_mr_file_changes config option and --no-file-changes CLI flag
- Add GitLab MR diffs API fetch pipeline with watermark-based sync
- Create migration 020 for diffs_synced_for_updated_at watermark column
- Rewrite query_expert() and query_overlap() to use 4-signal UNION ALL:
  DiffNote reviewers, DiffNote MR authors, file-change authors, file-change reviewers
- Deduplicate across signal types via COUNT(DISTINCT CASE WHEN ... THEN mr_id END)
- Add insert_file_change test helper, 8 new who tests, all 397 tests pass
- Also includes: list performance migration 019, autocorrect module, README updates

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/core/db.rs

@@ -57,6 +57,14 @@ const MIGRATIONS: &[(&str, &str)] = &[
         "018",
         include_str!("../../migrations/018_fix_assignees_composite_index.sql"),
     ),
+    (
+        "019",
+        include_str!("../../migrations/019_list_performance.sql"),
+    ),
+    (
+        "020",
+        include_str!("../../migrations/020_mr_diffs_watermark.sql"),
+    ),
 ];
 
 pub fn create_connection(db_path: &Path) -> Result<Connection> {