fix: scheduled task notifications not delivered on cancellation

GenerationCancelled extends BaseException, not Exception, so it bypassed
the except handler in _run_initial. The finally block ran but
_extract_last_assistant_content returned "" (response never appended to
messages), and _fire_notify_targets bailed on the empty content guard.

Fixes:
- Catch BaseException (not just Exception) in _run_initial so
  GenerationCancelled is handled and the UI state is cleaned up
- Remove the empty-content suppression in _fire_notify_targets —
  scheduled tasks should always deliver, even with a fallback message
  when no output was captured

tests/test_notify_completion.py

@@ -349,11 +349,14 @@ class TestFireNotifyTargets:
         mock_deliver.assert_not_called()
 
     @patch("turnstone.server._deliver_notification")
-    def test_empty_content_skipped(self, mock_deliver):
+    def test_empty_content_delivers_fallback(self, mock_deliver):
+        """Empty content should still deliver with a fallback message."""
         ws = MagicMock()
         ws.notify_targets = '[{"channel_type":"discord","channel_id":"1"}]'
         _fire_notify_targets(ws, "")
-        mock_deliver.assert_not_called()
+        mock_deliver.assert_called_once()
+        payload = mock_deliver.call_args[0][1]
+        assert "no output captured" in payload["message"]
 
     @patch("turnstone.server._deliver_notification")
     def test_invalid_json_targets_skipped(self, mock_deliver):

turnstone/server.py

@@ -1735,8 +1735,10 @@ def _extract_last_assistant_content(session: Any) -> str:
 
 def _fire_notify_targets(ws: Any, content: str) -> None:
     """Send completion notifications to all configured targets."""
-    if not content or not ws.notify_targets:
+    if not ws.notify_targets:
         return
+    if not content:
+        content = "(Task completed — no output captured)"
 
     try:
         targets = json.loads(ws.notify_targets)
@@ -2032,7 +2034,7 @@ async def create_workstream(request: Request) -> JSONResponse:
             def _run_initial() -> None:
                 try:
                     session.send(initial_message)
-                except Exception:
+                except BaseException:
                     if isinstance(ws.ui, WebUI):
                         ws.ui.on_stream_end()
                         ws.ui.on_state_change("idle")