Wire ChatAgent as first MemoryMixin consumer

[kovtcharov]

Summary

Integrate MemoryMixin into ChatAgent as the first consumer. This validates the mixin works end-to-end with a real agent.

Spec: docs/spec/agent-memory-architecture.md → Implementation Order step 6

Depends on: #542 (MemoryStore), #543 (MemoryMixin)

Changes

ChatAgent (src/gaia/agents/chat/agent.py)

class ChatAgent(Agent, MemoryMixin):
    def __init__(self, **kwargs):
        self.init_memory()          # Before super().__init__()
        super().__init__(**kwargs)

    def _register_tools(self):
        super()._register_tools()
        self.register_memory_tools()

Validation

• 5 memory tools appear in tool list
• System prompt includes memory section
• Conversations stored after each query
• Tool calls auto-logged
• Errors auto-learned
• Memory persists across sessions (restart agent, recall works)

Acceptance Criteria

• ChatAgent inherits MemoryMixin
• Memory tools available in gaia chat
• remember / recall / update_memory / forget work in conversation
• search_past_conversations returns past sessions
• System prompt shows preferences and facts
• Tool errors surface in system prompt on next query
• Memory persists across agent restarts