fix(hindsight): preserve shared event loop across provider shutdowns

The module-global `_loop` / `_loop_thread` pair is shared across every
`HindsightMemoryProvider` instance in the process — the plugin loader
creates one provider per `AIAgent`, and the gateway creates one `AIAgent`
per concurrent chat session (Telegram/Discord/Slack/CLI).

`HindsightMemoryProvider.shutdown()` stopped the shared loop when any one
session ended. That stranded the aiohttp `ClientSession` and `TCPConnector`
owned by every sibling provider on a now-dead loop — they were never
reachable for close and surfaced as the `Unclosed client session` /
`Unclosed connector` warnings reported in #11923.

Fix: stop stopping the shared loop in `shutdown()`. Per-provider cleanup
still closes that provider's own client via `self._client.aclose()`. The
loop runs on a daemon thread and is reclaimed on process exit; keeping
it alive between provider shutdowns means sibling providers can drain
their own sessions cleanly.

Regression tests in `tests/plugins/memory/test_hindsight_provider.py`
(`TestSharedEventLoopLifecycle`):

- `test_shutdown_does_not_stop_shared_event_loop` — two providers share
  the loop; shutting down one leaves the loop live for the other. This
  test reproduces the #11923 leak on `main` and passes with the fix.
- `test_client_aclose_called_on_cloud_mode_shutdown` — each provider's
  own aiohttp session is still closed via `aclose()`.

Fixes #11923.

plugins/memory/hindsight/__init__.py

@@ -1112,7 +1112,6 @@ class HindsightMemoryProvider(MemoryProvider):
 
     def shutdown(self) -> None:
         logger.debug("Hindsight shutdown: waiting for background threads")
-        global _loop, _loop_thread
         for t in (self._prefetch_thread, self._sync_thread):
             if t and t.is_alive():
                 t.join(timeout=5.0)
@@ -1131,13 +1130,17 @@ class HindsightMemoryProvider(MemoryProvider):
             except Exception:
                 pass
             self._client = None
-        # Stop the background event loop so no tasks are pending at exit
-        if _loop is not None and _loop.is_running():
-            _loop.call_soon_threadsafe(_loop.stop)
-            if _loop_thread is not None:
-                _loop_thread.join(timeout=5.0)
-            _loop = None
-            _loop_thread = None
+        # The module-global background event loop (_loop / _loop_thread)
+        # is intentionally NOT stopped here. It is shared across every
+        # HindsightMemoryProvider instance in the process — the plugin
+        # loader creates a new provider per AIAgent, and the gateway
+        # creates one AIAgent per concurrent chat session. Stopping the
+        # loop from one provider's shutdown() strands the aiohttp
+        # ClientSession + TCPConnector owned by every sibling provider
+        # on a dead loop, which surfaces as the "Unclosed client session"
+        # / "Unclosed connector" warnings reported in #11923. The loop
+        # runs on a daemon thread and is reclaimed on process exit;
+        # per-session cleanup happens via self._client.aclose() above.
 
 
 def register(ctx) -> None: