fix(auxiliary): custom provider URL rewrite + main_runtime model for title gen

- auxiliary_client: apply _to_openai_base_url() to custom base_url
  (fixes /anthropic → /v1 rewrite missing for provider="custom")
- auxiliary_client: use main_runtime.get("model") instead of _read_main_model()
  so auxiliary tasks follow system default model changes
- title_generator: thread main_runtime through generate_title → auto_title_session → maybe_auto_title
- cli.py / gateway/run.py: pass main_runtime to maybe_auto_title
- tests: update mock assertions for new main_runtime parameter

tests/agent/test_title_generator.py

@@ -182,7 +182,7 @@ class TestMaybeAutoTitle:
             import time
             time.sleep(0.3)
             mock_auto.assert_called_once_with(
-                db, "sess-1", "hello", "hi there", failure_callback=None
+                db, "sess-1", "hello", "hi there", failure_callback=None, main_runtime=None
             )
 
     def test_forwards_failure_callback_to_worker(self):
@@ -202,7 +202,7 @@ class TestMaybeAutoTitle:
             import time
             time.sleep(0.3)
             mock_auto.assert_called_once_with(
-                db, "sess-1", "hello", "hi there", failure_callback=_cb
+                db, "sess-1", "hello", "hi there", failure_callback=_cb, main_runtime=None
             )
 
     def test_skips_if_no_response(self):