feat(gateway): inject stable human-readable message timestamps

Consolidates these related Amy fork patches:
- 429830f39 feat(gateway): inject message timestamps into user messages for LLM context
- 3c3d6fac0 fix: handle both ISO string and epoch float timestamps in history replay
- 2874f7725 feat: human-friendly timestamp format with weekday and timezone name
- 3735f4c8b fix: render gateway message timestamps once

tests/gateway/test_session_api.py

@@ -241,7 +241,11 @@ async def test_session_chat_loads_history_and_preserves_session_headers(auth_ada
     assert kwargs["session_id"] == session_id
     assert kwargs["gateway_session_key"] == "client-42"
     assert kwargs["ephemeral_system_prompt"] == "stay focused"
-    assert kwargs["conversation_history"] == [
+    history = kwargs["conversation_history"]
+    assert len(history) == 2
+    assert isinstance(history[0].pop("timestamp"), (int, float))
+    assert isinstance(history[1].pop("timestamp"), (int, float))
+    assert history == [
         {"role": "user", "content": "earlier"},
         {"role": "assistant", "content": "prior answer"},
     ]