perf(me): optimize mentions query with materialized CTEs scoped to candidates

The `query_mentioned_in` SQL previously joined notes directly against
the full issues/merge_requests tables, with per-row subqueries for
author/assignee/reviewer exclusion. On large databases this produced
pathological query plans where SQLite scanned the entire notes table
before filtering to relevant entities.

Refactor into a dedicated `build_mentioned_in_sql()` builder that:

1. Pre-filters candidate issues and MRs into MATERIALIZED CTEs
   (state open OR recently closed, not authored by user, not
   assigned/reviewing). This narrows the working set before any
   notes join.

2. Computes note timestamps (my_ts, others_ts, any_ts) as separate
   MATERIALIZED CTEs scoped to candidate entities only, rather than
   scanning all notes.

3. Joins mention-bearing notes against the pre-filtered candidates,
   avoiding the full-table scans.

Also adds a test verifying that authored issues are excluded from the
mentions results, and a unit test asserting all four CTEs are
materialized.

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

src/cli/commands/me/me_tests.rs

@@ -880,6 +880,21 @@ fn mentioned_in_excludes_assigned_issue() {
     assert!(results.is_empty(), "should exclude assigned issues");
 }
 
+#[test]
+fn mentioned_in_excludes_authored_issue() {
+    let conn = setup_test_db();
+    insert_project(&conn, 1, "group/repo");
+    insert_issue(&conn, 10, 1, 42, "alice"); // alice IS author
+    let disc_id = 100;
+    insert_discussion(&conn, disc_id, 1, None, Some(10));
+    let t = now_ms() - 1000;
+    insert_note_at(&conn, 200, disc_id, 1, "bob", false, "hey @alice", t);
+
+    let recency_cutoff = now_ms() - 7 * 24 * 3600 * 1000;
+    let results = query_mentioned_in(&conn, "alice", &[], recency_cutoff).unwrap();
+    assert!(results.is_empty(), "should exclude authored issues");
+}
+
 #[test]
 fn mentioned_in_finds_mention_on_non_authored_mr() {
     let conn = setup_test_db();
@@ -1093,6 +1108,27 @@ fn mentioned_in_rejects_false_positive_email() {
 
 // ─── Helper Tests ──────────────────────────────────────────────────────────
 
+#[test]
+fn mentioned_in_sql_materializes_core_ctes() {
+    let sql = build_mentioned_in_sql("");
+    assert!(
+        sql.contains("candidate_issues AS MATERIALIZED"),
+        "candidate_issues should be materialized"
+    );
+    assert!(
+        sql.contains("candidate_mrs AS MATERIALIZED"),
+        "candidate_mrs should be materialized"
+    );
+    assert!(
+        sql.contains("note_ts_issue AS MATERIALIZED"),
+        "note_ts_issue should be materialized"
+    );
+    assert!(
+        sql.contains("note_ts_mr AS MATERIALIZED"),
+        "note_ts_mr should be materialized"
+    );
+}
+
 #[test]
 fn parse_attention_state_all_variants() {
     assert_eq!(