Forum thread mentions never reach managed agents

**Describe the bug**

A managed agent that is a member of a forum channel never responds to an `@mention` posted inside a forum thread. It stays silent, logs nothing, and reports no error, so the agent looks perfectly healthy while ignoring every mention.

The cause is the subscription filter. In `crates/buzz-acp/src/config.rs`, `SubscribeMode::Mentions` builds a fixed kind list:

```rust
let kinds = config.kinds_override.clone().unwrap_or_else(|| {
vec![KIND_STREAM_MESSAGE, KIND_WORKFLOW_APPROVAL_REQUESTED, KIND_STREAM_REMINDER]
});
```

That is `9`, `46010`, `40007`. Forum posts (`45001`) and forum comments (`45003`) are absent. In `crates/buzz-acp/src/filter.rs` an event whose kind is not in the list is rejected before the `require_mention` p-tag check ever runs, so the mention is discarded at the transport layer.

Neither forum kind appears anywhere in the `buzz-acp` crate.

**Steps to reproduce**

1. Create a forum channel and add a managed agent to it as a bot
2. Create a thread and mention the agent inside it
3. The agent never wakes up. Its log shows no incoming event.
4. Mention the same agent in a stream channel: it responds normally

**Expected behavior**

Mentioning an agent in a forum thread should reach it, same as in a stream channel. The UI lets you add agents to forum channels, so the mention path should work there.

**Workaround**

Setting the environment variable below restores it. The variable replaces the default list rather than extending it, so the three original kinds have to be repeated:

```
BUZZ_ACP_KINDS=9,40007,46010,45001,45003
```

This is safe: `require_mention` stays enabled and its p-tag check is kind-agnostic, so agents still only wake when tagged.

**Suggested fix**

Add `KIND_FORUM_POST` and `KIND_FORUM_COMMENT` to the `SubscribeMode::Mentions` defaults in both places they are constructed (`build_channel_filters` and the dynamic single-channel path). Routing on the p-tag rather than on kind would close the whole class of problem, but adding the two kinds is the minimal change.

**Version and platform**

- Buzz version: 0.5.6
- OS: Windows 11 (10.0.26200)

**Additional context**

Two follow-on effects are visible once mentions do arrive, filed separately: typing indicators and the persistent agent audience are both stream-only in the desktop UI.

Start in crates/buzz-acp/src/config.rs at SubscribeMode::Mentions and compare the defaults in build_channel_filters with the dynamic single-channel path; then inspect crates/buzz-acp/src/filter.rs to confirm the kind filter precedes require_mention. Add the two forum kinds to both defaults and verify forum-thread mentions arrive while existing stream kinds and p-tag filtering still work.

Written by the indexing model from the issue text.