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/config.rs

@@ -49,6 +49,9 @@ pub struct SyncConfig {
 
     #[serde(rename = "fetchResourceEvents", default = "default_true")]
     pub fetch_resource_events: bool,
+
+    #[serde(rename = "fetchMrFileChanges", default = "default_true")]
+    pub fetch_mr_file_changes: bool,
 }
 
 fn default_true() -> bool {
@@ -66,6 +69,7 @@ impl Default for SyncConfig {
             dependent_concurrency: 8,
             requests_per_second: 30.0,
             fetch_resource_events: true,
+            fetch_mr_file_changes: true,
         }
     }
 }