Copy-paste loses agent mention formatting

## Bug Report

When copying a message containing agent @mentions and pasting it elsewhere, the mention formatting is sometimes lost, causing the agents to not be invoked when the message is sent.

### Steps to Reproduce

1. Copy a message containing one or more agent @mentions
2. Paste the message into the composer
3. Send the message
4. Observe whether the mentioned agents are notified

### Observed Behavior

The pasted content sometimes loses the special mention formatting that triggers agent invocation. Agents mentioned in the pasted message are not notified and do not respond, even though they appear as "@Name" in the text.

### Expected Behavior

Copy-paste should preserve mention formatting. When a message with "@AgentName" is pasted and sent, the agent should be notified and respond (if configured to do so). The mention should be preserved as a proper `p` tag in the Nostr event.

### Environment

- Buzz Desktop (copy-paste source)
- Messages with agent mentions

### Related

- Related to #2319 (NIP-27 mentions) — proper NIP-27 encoding in the clipboard would ensure mentions survive copy-paste
- Related to agent mention delivery issues

Start by tracing clipboard handling in Buzz Desktop and the NIP-27 mention behavior referenced in the issue. Verify that pasted agent mentions retain proper p-tag formatting in the Nostr event and that sending the pasted message notifies the mentioned agents.

Written by the indexing model from the issue text.