fix(desktop): offer shared relay agents in @ mention autocomplete (#3971)

[iroiro147]

Summary

Fixes #3971 — the @ mention autocomplete never offered remotely-hosted agents, even when they were channel members, appeared in the members panel, and were registered in the relay agent directory with respond_to=anyone.

Root cause

In desktop/src/features/messages/lib/useMentions.ts, addCandidate dropped any isAgent candidate not in the local managed-agent list:

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

This ran before shouldHideAgentFromMentions, whose mentionableAgentPubkeys set (built from relayAgentIsSharedWithUser — directory agents that share a channel with the user and advertise respond_to=anyone or an allowlist match) is precisely the machinery meant to make shared relay agents mentionable. The early local-only filter made that set unreachable dead code for any agent hosted on another machine.

Fix

Admit the candidate when it is either locally managed or in the relay-mentionable set:

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

shouldHideAgentFromMentions still runs downstream and independently consults mentionableAgentPubkeys, so noise-gating is preserved: a remote agent that is not relay-shared with the user is still dropped.

Verification

• pnpm typecheck — clean
• pnpm exec biome check — clean (both touched files)
• node --import ./test-loader.mjs --experimental-strip-types --test src/features/agents/lib/agentAutocompleteEligibility.test.mjs — 20/20 pass (18 baseline + 2 new pinning tests)
• Mention suites (mentionCandidates.test.mjs, mentionRanking.test.mjs) — green

Tests added

agentAutocompleteEligibility.test.mjs, pinning the new admit condition:

• a shared relay agent not in the local managed list is admitted;
• a non-shared non-local agent is still rejected.

Impact

Multi-device / team setups where an agent runs on one machine and users chat from another now get real @Name autocomplete → p-tag delivery. Workarounds (thread replies to p-tag the parent author, agent-side content-matching rules) are no longer needed.