session/Cleanup: remove stale entries from vector index

Store.Cleanup() primary path (with index) bypassed Store.Delete() and
directly removed files + index entries — but never called Vec.Remove().

This left orphaned vectors in the vector store on every /prune command.
They were silently skipped during search (Load fails -> continue), but
accumulated garbage in vectors.gob.

Now calls Vec.Remove(id) alongside file removal, matching the fallback
path (which correctly used Store.Delete() -> Vec.Remove()).

Author: molty3000

internal/session/session.go

@@ -500,6 +500,10 @@ func (s *Store) Cleanup(before time.Time) (int, error) {
 					return deleted, fmt.Errorf("session: delete %q: %w", id, err)
 				}
 				delete(idx, id)
+				// Remove from vector index to prevent stale entries.
+				if s.Vec != nil {
+					_ = s.Vec.Remove(id) // best-effort
+				}
 				deleted++
 			}
 		}