Implement trace store

[dgenio]

Summary

In-memory store for action traces, enabling the explain() API.

API

• TraceStore.record(trace: ActionTrace) → None
• TraceStore.get(action_id: str) → ActionTrace — raise if not found
• TraceStore.list(limit: int | None = None) → list[ActionTrace] — most recent first

Acceptance criteria

• Record + get round-trip
• Missing action_id raises error
• list returns most recent first
• Traces are immutable after recording (modifying the returned object doesn't affect the store)