Mention autocomplete never offers remotely-hosted agents (shared relay agent eligibility is unreachable)

**Version:** 0.5.2 (also present on current main)

**Setup:** An ACP agent hosted on machine A (via buzz-acp with NIP-OA owner attestation), and Buzz Desktop on machine B owned by the same user. The agent is a member of a shared channel, publishes a kind-0 profile, appears in the member list with the agent chip ("managed by you"), is registered in the relay agent directory, and advertises `respond_to=anyone`.

**Bug:** The `@` mention autocomplete on machine B never offers the agent, so there is no way to send it a p-tag mention from the UI. The members panel shows it; the composer refuses to.

**Root cause:** In `desktop/src/features/messages/lib/useMentions.ts`, `addCandidate` drops any candidate flagged `isAgent` that is not in the *local* managed-agent list:

if (!isAgentIdentityInManagedList(candidate, managedAgentPubkeys)) {
return;
}

This runs *before* `shouldHideAgentFromMentions`, whose `mentionableAgentPubkeys` set (built from `relayAgentIsSharedWithUser` — directory agents with `respond_to=anyone`/allowlist sharing a channel) is meant to make exactly these agents mentionable. Because the earlier filter only consults the local list, that machinery is unreachable dead code for any agent hosted on another machine.

**Suggested fix:** let mentionable relay agents through the first filter:

if (
!isAgentIdentityInManagedList(candidate, managedAgentPubkeys) &&
!mentionableAgentPubkeys.has(pubkey)
) {
return;
}

**Impact:** Any multi-device / team setup where an agent runs on one machine and users chat from another. Current workarounds are thread replies (reply p-tags the parent author) or agent-side content-matching rules instead of real mentions.

Start in desktop/src/features/messages/lib/useMentions.ts at addCandidate, then trace how mentionableAgentPubkeys is built and how shouldHideAgentFromMentions uses it. Verify that a remotely hosted shared relay agent with respond_to=anyone appears in @ autocomplete and can be mentioned from the other device.

Written by the indexing model from the issue text.