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>

migrations/019_list_performance.sql

@@ -0,0 +1,13 @@
+-- Standalone updated_at DESC indexes for ORDER BY without temp B-tree sort.
+-- The existing composite indexes (project_id, updated_at) only help when
+-- filtering by project first.
+CREATE INDEX IF NOT EXISTS idx_issues_updated_at_desc
+    ON issues(updated_at DESC);
+
+CREATE INDEX IF NOT EXISTS idx_mrs_updated_at_desc
+    ON merge_requests(updated_at DESC);
+
+-- Covering index for correlated subquery: unresolved discussion count per issue.
+-- MRs already have idx_discussions_mr_resolved (migration 006).
+CREATE INDEX IF NOT EXISTS idx_discussions_issue_resolved
+    ON discussions(issue_id, resolvable, resolved);

migrations/020_mr_diffs_watermark.sql

@@ -0,0 +1,7 @@
+-- Migration 020: Watermark column for MR diffs sync
+-- Tracks which MRs have had their file changes fetched, same pattern as closes_issues_synced_for_updated_at
+
+ALTER TABLE merge_requests ADD COLUMN diffs_synced_for_updated_at INTEGER;
+
+INSERT INTO schema_version (version, applied_at, description)
+VALUES (20, strftime('%s', 'now') * 1000, 'MR diffs sync watermark');