feat(me): add 30-day mention age cutoff to filter stale @-mentions

Previously, query_mentioned_in returned mentions from any time in the
entity's history as long as the entity was still open (or recently closed).
This caused noise: a mention from 6 months ago on a still-open issue would
appear in the dashboard indefinitely.

Now the SQL filters notes by created_at > mention_cutoff_ms, defaulting to
30 days. The recency_cutoff (7 days) still governs closed/merged entity
visibility — this new cutoff governs mention note age on open entities.

Signature change: query_mentioned_in gains a mention_cutoff_ms parameter.
All existing test call sites updated. Two new tests verify the boundary:
- mentioned_in_excludes_old_mention_on_open_issue (45-day mention filtered)
- mentioned_in_includes_recent_mention_on_open_issue (5-day mention kept)

src/cli/commands/me/queries.rs

@@ -849,6 +849,7 @@ fn build_mentioned_in_sql(project_clause: &str) -> String {
         LEFT JOIN note_ts_issue nt ON nt.issue_id = ci.id
         WHERE n.is_system = 0
           AND n.author_username != ?1
+          AND n.created_at > ?3
           AND LOWER(n.body) LIKE '%@' || LOWER(?1) || '%'
         UNION ALL
         -- MR mentions (scoped to candidate entities only)
@@ -862,6 +863,7 @@ fn build_mentioned_in_sql(project_clause: &str) -> String {
         LEFT JOIN note_ts_mr nt ON nt.merge_request_id = cm.id
         WHERE n.is_system = 0
           AND n.author_username != ?1
+          AND n.created_at > ?3
           AND LOWER(n.body) LIKE '%@' || LOWER(?1) || '%'
         ORDER BY 6 DESC
         LIMIT 500",
@@ -871,7 +873,8 @@ fn build_mentioned_in_sql(project_clause: &str) -> String {
 /// Query issues and MRs where the user is @mentioned but not assigned/authored/reviewing.
 ///
 /// Includes open items unconditionally, plus recently-closed/merged items
-/// (where `updated_at > recency_cutoff_ms`).
+/// (where `updated_at > recency_cutoff_ms`). Only considers mentions in notes
+/// created after `mention_cutoff_ms` (typically 30 days ago).
 ///
 /// Returns deduplicated results sorted by attention priority then recency.
 pub fn query_mentioned_in(
@@ -879,14 +882,16 @@ pub fn query_mentioned_in(
     username: &str,
     project_ids: &[i64],
     recency_cutoff_ms: i64,
+    mention_cutoff_ms: i64,
 ) -> Result<Vec<MeMention>> {
-    let project_clause = build_project_clause_at("p.id", project_ids, 3);
+    let project_clause = build_project_clause_at("p.id", project_ids, 4);
     // Materialized CTEs avoid pathological query plans for project-scoped mentions.
     let sql = build_mentioned_in_sql(&project_clause);
 
     let mut params: Vec<Box<dyn rusqlite::types::ToSql>> = Vec::new();
     params.push(Box::new(username.to_string()));
     params.push(Box::new(recency_cutoff_ms));
+    params.push(Box::new(mention_cutoff_ms));
     for &pid in project_ids {
         params.push(Box::new(pid));
     }