fix: include active root sessions in Recent sidebar

Recent now shows active root sessions immediately, even if they are older than 48 hours
Archived sessions and subtasks still stay out of Recent
Added coverage and documentation for the new recency rules

packages/ui/src/components/session/sidebar/activitySections.ts

@@ -22,20 +22,19 @@ const getSessionUpdatedAt = (session: Session): number => {
   return 0;
 };
 
-// Recent sessions are simply every non-archived, top-level session updated
-// within the last RECENT_SESSION_MAX_AGE_MS. No persisted history or live-busy
-// tracking: membership is timestamp-derived, while the caller applies shared
-// lifecycle ordering.
+// Recent contains non-archived root sessions that are active now or were
+// updated within the retention window. The caller applies shared lifecycle
+// ordering after this membership filter.
 export const deriveRecentSessions = (
   sessions: Session[],
+  activeSessionIds: ReadonlySet<string>,
   now = Date.now(),
 ): Session[] => {
   const minUpdatedAt = now - RECENT_SESSION_MAX_AGE_MS;
-  const recent = sessions.filter((session) => {
+  return sessions.filter((session) => {
     if (isArchivedSession(session) || isSubtaskSession(session)) {
       return false;
     }
-    return getSessionUpdatedAt(session) >= minUpdatedAt;
+    return activeSessionIds.has(session.id) || getSessionUpdatedAt(session) >= minUpdatedAt;
   });
-  return recent;
 };