Desktop: agents owned by another user are never mentionable — the managed-list guard in useMentions dead-codes shouldHideAgentFromMentions

Summary

On Buzz Desktop, an agent deployed by user A is invisible in user B's @-mention autocomplete — even when the agent is a member of the same channel and B is on the agent's respond_to allowlist. Typing the name by hand doesn't help either: the message is sent without a p tag, so the agent's harness never triggers.

This makes cross-user agent collaboration (the "agents are members, not bots" story from the README) work on mobile but not on desktop.

Root cause

In desktop/src/features/messages/lib/useMentions.ts, addCandidate runs two gates in sequence:

if (!isAgentIdentityInManagedList(candidate, managedAgentPubkeys)) return;   // gate 1
if (shouldHideAgentFromMentions({ ..., mentionableAgentPubkeys, ... })) return; // gate 2

1. Gate 1 drops every isAgent candidate whose pubkey is not in the local managed-agent list (managed-agents.json). On any install that doesn't manage agents itself, this is the empty set — every agent candidate dies here, including channel members with role bot.
2. Gate 2 (shouldHideAgentFromMentions) starts with if (mentionableAgentPubkeys.has(normalized)) return false; // Invocable => always show.
3. getMentionableAgentPubkeys seeds its result with managedAgentPubkeys (agentAutocompleteEligibility.ts), so managedAgentPubkeys ⊆ mentionableAgentPubkeys always holds.

⟹ Every agent candidate that survives gate 1 is already "invocable" to gate 2, which therefore returns false on its first line, always. All remaining branches of shouldHideAgentFromMentions — including the "Option B" member branch (lines 84–97) whose comment block explicitly specifies "Member: … Unknown invocability (not in directory) => show" — are unreachable from their only production call site. The policy documented there was designed for exactly this scenario, and gate 1 starves it.

Evidence this is a regression, not intent

• The mobile client is the witness. mobile/lib/features/channels/mentions/mention_candidates.dart implements the same pipeline without the pre-gate (member bots are always candidates; agentIsSharedWithUser handles non-member directory agents), and its comments say "Mirrors desktop's shouldHideAgentFromMentions". The port codified desktop's intended semantics; desktop no longer executes them.
• Coverage has a hole exactly here. The e2e suite pins "relay-only (non-member) agents stay hidden" and "own managed agents are visible" — there is no test for channel-member agent owned by another user.
• The typed-mention fallback is also starved. extractMentionPubkeys has an explicit loop resolving hand-typed @Name against channel members without autocomplete. It works for humans; for foreign agents, gate 1 has already removed them from mentionCandidates.

Net effect: on a desktop with an empty managed list, no agent is mentionable at all — not even one with respond_to: anyone. The client-side allowlist/anyone logic (relayAgentIsSharedWithUser, getMentionableAgentPubkeys) is unreachable ornament.

isAgentIdentityInManagedList has a legitimate job — its unit test says "keeps people and only current managed agent identities", i.e. dropping stale/ghost agent identities surfaced by discovery/search. The bug is only that it also swallows relay-confirmed channel members.

Fix

Exempt channel members from gate 1 so that gate 2 — which already encodes the correct member policy — actually decides:

if (
  candidate.isMember !== true &&
  !isAgentIdentityInManagedList(candidate, managedAgentPubkeys)
) {
  return;
}

Ghost-guard semantics are preserved for non-members (discovery, directory, global search), so the pinned "relay-only agents stay hidden" behavior is unchanged.

PR with the fix and an e2e regression test (channel-member bot + empty managed list ⇒ visible in @, mention resolves to a p tag on the wire): #TBD

Repro

1. User A deploys an agent and adds it to a shared channel (member role bot).
2. User B (no locally managed agents) opens the channel on desktop and types @.
3. The agent is missing from the list; typing @AgentName by hand sends a message without any p tag (verifiable in the event), so the agent never responds.
4. Same account, same channel on mobile: the agent appears and the mention works.