buzz-acp does not start agent turns for p-tagged mentions in forum thread replies

## Summary

Agent mentions in Buzz forum thread replies are visible on the relay and in `buzz feed get`, but they do not start an ACP harness turn. The same agent can be reached successfully from a DM, so the harness is alive and authenticated; the failure appears specific to forum thread reply events.

## Impact

This prevents using forum threads as the main work surface for agent collaboration. Users can add the agent as a channel member and tag it in a forum thread, but the agent never responds because the ACP session is not started.

## Observed behavior

- A forum/root post exists in `#Music Player` as kind `45001`.
- Replies in that forum thread are kind `45003`.
- A reply containing `@Zoidberg` was published from Buzz Desktop on the Mac mini.
- The reply event includes:
- `h` tag for the forum channel: `43790c7a-5947-4ab7-9460-c2542cecc4dc`
- `e` tag pointing at the forum root: `6ab540744c212f7bc9ad38f0aacd5a8cc052792c8d4fb032d370aafd28d209ea`
- `p` tag for the live agent identity: `e93c449f2aef397fd758875e280ad67cd8098f19d96e3a3e3f06c9b89d8e1740`
- The event appears in `buzz feed get` for the agent environment.
- No ACP turn/session starts and the agent does not reply.

## Expected behavior

When a forum thread reply is p-tagged with an agent identity that is a member of the channel, `buzz-acp` should treat it like an actionable mention and start a turn for that agent.

The started turn should include enough thread context for the agent to reply back into the forum thread, including the channel UUID and correct reply/root event IDs.

## Reproduction steps

1. Create or open a Buzz forum channel.
2. Add an ACP-backed agent as a bot/member of that forum channel.
3. Create a forum post/root thread.
4. Reply inside the forum thread and mention the agent.
5. Verify the reply event is kind `45003` and includes the agent `p` tag.
6. Verify `buzz feed get` can see the event.
7. Observe that no ACP turn starts for the agent.

## Notes from local debugging

DM mentions to the same agent work. The failing event was visible in the feed and correctly p-tagged, so this does not look like a missing channel membership or bad mention-resolution issue anymore.

This may be a kind-filter/routing gap where the ACP harness handles kind `9` channel/DM mentions but does not treat kind `45003` forum-thread replies as actionable mentions.

## Related but separate issues noticed during debugging

- `buzz channels add-member` from a managed agent environment failed with relay error `invalid: missing p tag`, even when `--pubkey` was supplied.
- Buzz Desktop can become confusing when two owner-owned agents share the same display name/avatar and one is archived; one client appeared to resolve only the archived identity.