Desktop: disabled “Automatically mention agents” still pins agents in threads

## Bug description

Buzz Desktop continues pinning agents for automatic mentions in a thread even when **Automatically mention agents** is disabled.

Both controls show the feature as off:

- the thread composer's automatic-mention popover;
- Settings → Agents → Automatically mention agents.

The live persisted preference also contains:

```text
buzz.messages.keepMentionedAgentsPinned = false
```

Despite that, explicitly mentioning an agent once activates it for subsequent replies. The next composer shows the agent as automatically selected and says that the agent will be mentioned automatically. This reproduces with different agents.

## Steps to reproduce

1. Open Buzz Desktop and go to **Settings → Agents**.
2. Turn **Automatically mention agents** off.
3. Open a channel thread.
4. Explicitly mention an agent in a reply and send it.
5. Inspect the composer after the message sends.

## Expected behavior

With the setting off, the explicit mention is one-shot. After sending, the next composer is empty and no agent is selected for automatic mention.

## Actual behavior

The explicitly mentioned agent remains activated in the next composer and is automatically mentioned in later replies, even though both UI controls still show the setting as off and the persisted preference is `false`.

Removing the agent from the composer is only temporary: explicitly mentioning that agent—or another agent—activates automatic mentioning again.

## Environment

- Buzz Desktop: 0.5.20
- macOS: 26.5.2 (25F84)

## Missing regression coverage

The relevant acceptance case is:

```text
automatic mentions off → explicitly mention agent → send → next composer has no pinned agent
```

Existing persistent-audience coverage exercises enabled persistence and one-shot timeline behavior, but does not cover this disabled thread-composer path.