buzz-acp: subscribe=mentions never receives forum-post mentions (kinds 45001/45003)

## Summary

Agents running under `buzz-acp` with the default `--subscribe mentions` mode never receive @mentions posted in **forum-style channels**. Forum posts are published as **kind `45001`** (and related forum events as **kind `45003`**), while the mentions subscription only appears to cover chat stream messages (**kind `9`**). The agent looks online and is a channel member, but never reacts to forum mentions.

## Environment

- Relay: `ghcr.io/block/buzz:main`, self-hosted via the Railway template (deployed 2026-08-07)
- Buzz Desktop v0.5.5 (macOS, Apple Silicon) managed agents — bundled `buzz-acp`, Claude harness (`claude-agent-acp` 0.66.0)
- Also observed with `buzz-acp` built from `main` (2026-08-07)

## Reproduction

1. Create a forum-type channel; add a managed agent as a channel member.
2. Agent harness logs the subscription:
```
INFO buzz_acp: membership notification: subscribing to new channel channel_id=0fcf197f-…
```
3. Create a **new** forum post mentioning the agent (post created after the subscription, so it is not a history/replay issue).
4. Agent's harness log shows **no inbound event at all** — no turn starts, no reply. (Log ends at the membership notification line.)

## Evidence

`buzz messages get` on the forum channel shows the kind split:

```
kind: 45001 | | "Vidiš li ovu poruku @Bumble ?" ← forum post, mention, never delivered to agent
kind: 45001 | | "@Bumble komentiraj nešto…" ← forum post, mention, never delivered to agent
kind: 45003 | | "@Arha @Argus @Bumble @Honey" ← forum event, same
kind: 9 | | reply from an agent running subscribe=all
```

A second agent connected to the same relay with `--subscribe all --no-mention-filter` **does** receive these forum events and can reply into the thread (its replies land as kind `9`), so relay-side delivery works — the gap is the kind filter in mentions mode.

## Expected

A mention inside a forum post should wake the agent the same way a kind:9 chat mention does (or, if forums are intentionally out of scope for mentions mode, the limitation deserves a note in the buzz-acp README).

Happy to provide more logs if useful. Great project — filed with ❤️ from a two-human, four-agent relay.