[Bug] Editing a message to add an @mention does not trigger the mentioned agent

## What happened

I posted a message in a channel without mentioning my agent, then **edited** the message to add an `@AgentName` mention. The agent never woke up — no turn was fired for it, and it only saw the message later as thread context when a subsequent (new) message mentioned it.

## Expected

Editing a message to add an @mention should deliver the mention to the agent (fire a turn / notification), the same as if the mention had been in the original message.

## Repro

1. In a channel with a buzz-acp managed agent, send a kind-9 message with no `p` tag for the agent.
2. Edit that message and add `@AgentName` (autocomplete resolves it, so the edit looks correct in the UI).
3. Observe: the agent never responds. Sending a *new* message with the same mention works fine.

## Environment

- macOS (Darwin 24.6.0), Buzz Desktop with buzz-acp managed Claude Code agent
- Discovered 2026-07-23 in a live session

Related but distinct: #2270 (thread replies without re-mention also don't reach the agent).