feat(agents): enforce one librarian timeout budget

- add LIBRARIAN_TIMEOUT config (default 180s) and enforce it with
  asyncio.wait_for inside delegate_to_librarian, covering the live
  paths (steward direct delegation and SSE streaming) that had no cap
- timeouts fail honestly: success=False with a curated butler sentence,
  detail in logs
- set an explicit timeout on TatlockOllamaProvider's AsyncOpenAI client
  from OLLAMA_TIMEOUT instead of the SDK default (~600s per LLM call)
- remove the contradictory unused 60s default from
  AgentRequest.timeout_seconds; coordination falls back to the
  configured budget

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/agents/test_protocol.py

@@ -27,7 +27,8 @@ class TestAgentRequest:
 
         assert request.task == "Find information about Docker"
         assert request.context == ""
-        assert request.timeout_seconds == 60
+        # No hardcoded default - None defers to the configured budget
+        assert request.timeout_seconds is None
 
     def test_request_with_context(self):
         """Test request with additional context."""