"Keep addressed agents active" has no effect in forum threads

**Describe the bug**

The Settings toggle "Keep addressed agents active" describes itself as:

> Keep agents you address selected for future messages in the same channel **or thread**. Remove them from the composer at any time.

The thread half of that promise is not implemented. In a forum thread the selection never persists, so every follow-up message needs the `@mention` typed again. In a stream channel the toggle behaves exactly as described.

The implementation is confined to the stream composer:

- `desktop/src/features/messages/lib/persistentAgentAudience.ts`
- `desktop/src/features/messages/ui/usePersistentAgentMentionHydration.ts`
- `desktop/src/features/messages/ui/MessageComposer.tsx`

`ForumComposer.tsx` is a separate component and references none of it. It does support `@` autocomplete, so mentions are typeable, but the audience is not retained after sending.

**Steps to reproduce**

1. Enable "Keep addressed agents active" in Settings, Agents
2. In a stream channel, mention an agent and send. Send a second message without typing the mention: it still reaches the agent
3. In a forum thread, mention an agent and send. Send a second message without typing the mention: nothing reaches the agent, and the composer shows no retained audience

**Expected behavior**

Either the toggle persists the audience in forum threads as its description states, or the description is corrected to say it applies to stream channels only. The current wording promises behaviour that cannot occur.

**Version and platform**

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

**Additional context**

Part of a pattern where the forum view lacks agent affordances the stream view has. Typing indicators are filed separately.