feat(responses): pass conversation context and stream thinks in real time

- delegate_to_* now receives a trimmed conversation history (last ~6
  turns, 500 chars/turn) as context on both live direct-delegation
  paths (streaming and steward non-streaming), via new
  build_delegation_context helper
- _stream_direct_delegation restructured as an async generator: the
  butler 'start' think message streams BEFORE the expert runs and the
  success/error message right after it finishes, instead of all
  messages arriving after the research completed

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

tests/agents/test_delegation.py

@@ -15,11 +15,69 @@ from src.agents.delegation import (
     DelegationResult,
     DelegationTask,
     _detect_action_type,
+    build_delegation_context,
     delegate_to_librarian,
     get_think_message,
 )
 
 
+@pytest.mark.unit
+class TestBuildDelegationContext:
+    """Tests for trimming conversation history into expert context."""
+
+    def test_empty_history_returns_empty(self):
+        assert build_delegation_context(None) == ""
+        assert build_delegation_context([]) == ""
+
+    def test_recent_turns_are_formatted(self):
+        history = [
+            {"role": "user", "content": "Tell me about Docker"},
+            {"role": "assistant", "content": "Docker is a container runtime."},
+        ]
+
+        context = build_delegation_context(history)
+
+        assert "Recent conversation:" in context
+        assert "user: Tell me about Docker" in context
+        assert "assistant: Docker is a container runtime." in context
+
+    def test_only_last_max_turns_kept(self):
+        history = [
+            {"role": "user", "content": f"message {i}"} for i in range(10)
+        ]
+
+        context = build_delegation_context(history, max_turns=6)
+
+        assert "message 3" not in context
+        assert "message 4" in context
+        assert "message 9" in context
+
+    def test_long_turns_are_truncated(self):
+        history = [{"role": "user", "content": "x" * 2000}]
+
+        context = build_delegation_context(history, max_chars_per_turn=500)
+
+        assert "x" * 500 in context
+        assert "x" * 501 not in context
+
+    def test_structured_content_parts_tolerated(self):
+        history = [
+            {"role": "user", "content": [{"type": "text", "text": "hello there"}]}
+        ]
+
+        context = build_delegation_context(history)
+
+        assert "hello there" in context
+
+    def test_non_dict_entries_skipped(self):
+        history = ["garbage", {"role": "user", "content": "real message"}]
+
+        context = build_delegation_context(history)
+
+        assert "real message" in context
+        assert "garbage" not in context
+
+
 @pytest.mark.unit
 class TestDelegationTask:
     """Tests for the DelegationTask dataclass."""