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

gateway/session.py

@@ -1322,6 +1322,7 @@ class SessionStore:
                         message.get("platform_message_id") or message.get("message_id")
                     ),
                     observed=bool(message.get("observed")),
+                    timestamp=message.get("timestamp"),
                 )
             except Exception as e:
                 logger.debug("Session DB operation failed: %s", e)