Desktop: selecting multiple agents from @mention autocomplete can omit a p tag

[redgun524]

Bug description

In a multi-person channel, a user selected two agent identities from the Desktop @mention autocomplete. The sent kind:9 event rendered both @Name strings, but its signed tags contained a p tag for only one selected agent. The omitted agent was not notified or invoked.

This is distinct from #6061: the user did explicitly select the autocomplete result.

Steps to reproduce

1. Open a multi-person channel/thread in Buzz Desktop.
2. Type @ and select Agent A from the autocomplete list.
3. Type @ again and select Agent B from the autocomplete list.
4. Send the message.
5. Inspect the published kind:9 event tags.

Expected behavior

The event contains one p tag for each selected mention, and both agents are notified/invoked.

Actual behavior

The rendered content contains both @Name strings, but only one selected agent appears in the signed p tags.

Evidence

Reproduced on 2026-09-08. A concrete event is available privately; its content had two selected agent mentions while tags contained only one corresponding p entry. The affected channel/event identifiers are intentionally omitted from this public issue because they belong to a private company workspace.

Environment

• Client: Buzz Desktop
• Exact app version: not yet captured
• OS: macOS

[redgun524]

Opened prematurely without completing duplicate/ownership triage. Closing.