- Add auto_skill field to MessageEvent for topic-skill bindings
- Gateway auto-loads the bound skill on new sessions via _load_skill_payload
- Skill content is injected into the first message (same as /skill commands)
- Subsequent messages in the session see it in conversation history
- Clean chat_topic (no [skill: ...] suffix) — skill flows via auto_skill field
- Add 3 tests for _build_message_event auto_skill behavior