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/gitlab/client.rs

@@ -12,7 +12,7 @@ use tracing::{debug, warn};
 
 use super::types::{
     GitLabDiscussion, GitLabIssue, GitLabIssueRef, GitLabLabelEvent, GitLabMergeRequest,
-    GitLabMilestoneEvent, GitLabProject, GitLabStateEvent, GitLabUser, GitLabVersion,
+    GitLabMilestoneEvent, GitLabMrDiff, GitLabProject, GitLabStateEvent, GitLabUser, GitLabVersion,
 };
 use crate::core::error::{LoreError, Result};
 
@@ -609,6 +609,15 @@ impl GitLabClient {
         self.fetch_all_pages(&path).await
     }
 
+    pub async fn fetch_mr_diffs(
+        &self,
+        gitlab_project_id: i64,
+        iid: i64,
+    ) -> Result<Vec<GitLabMrDiff>> {
+        let path = format!("/api/v4/projects/{gitlab_project_id}/merge_requests/{iid}/diffs");
+        coalesce_not_found(self.fetch_all_pages(&path).await)
+    }
+
     pub async fn fetch_issue_state_events(
         &self,
         gitlab_project_id: i64,