Desktop: shared relay agents never appear in mention autocomplete (mobile works)

### Describe the bug

On desktop, a shared/remote relay agent — one whose kind:10100 directory record makes it mention-eligible (`respond_to: "anyone"` + a shared channel) — never appears in the `@` mention autocomplete, so it cannot be selected as a pill. The same agent gets a pill on mobile immediately. Plaintext fallback matching (where the agent's ACP config accepts literal `@Name`) still works, which makes the failure look intermittent to users: typed text answers, the picker doesn't.

### To reproduce

1. Self-host a relay. Run an agent via `buzz-acp` on a separate machine (any host that isn't the desktop device), with a rich kind:10100 directory record: `respond_to: "anyone"`, `channel_ids` including a channel the desktop user has joined.
2. Add the agent to that channel (role `bot`). Confirm it authenticates and appears in the relay directory.
3. Desktop app: open the channel, type `@` — the agent is missing from the suggestion list.
4. Mobile app, same account, same channel: the agent appears and is selectable.

### Expected behavior

Directory-eligible shared agents should be offered as mention pills on desktop, matching mobile and matching desktop's own directory-based eligibility in `useNewMessageRecipients` and `ProjectsAgentPromptPage`.

### Mechanism (as of `3d7712c`)

`addCandidate()` in `desktop/src/features/messages/lib/useMentions.ts` calls `isAgentIdentityInManagedList()` **before** `shouldHideAgentFromMentions()`. Any `isAgent` candidate whose pubkey is not in the local managed-agents list is dropped — including every candidate produced by the `relayAgentsQuery` loop, which makes that loop dead code for non-local agents.

A remote agent can never satisfy that gate: `apply_inbound_managed_agent()` (`desktop/src-tauri/src/commands/personas/inbound.rs`) is patch-only by design ("managed agents are never minted from a relay event"), so no directory record, kind:30177 declaration, or channel membership ever creates a local managed entry on another device.

Two side observations while diagnosing, in case they're intentional-vs-known:

- `shouldHideAgentFromMentions`' member branch ("Member … unknown invocability => show") is unreachable at its only call site, because the pre-gate already requires mentionable-set membership. A channel-member agent with no usable directory record is hidden on desktop (mobile shows it).
- `MembersSidebar`'s add-member search uses the same managed-list gate and likely has the same remote-agent blindness — left untouched by the fix below since the UX semantics differ.

### Trust-model note

Directory eligibility (`respond_to` + shared `channel_ids`) is self-published per-pubkey, but this exact eligibility set is already trusted by mobile mentions, `useNewMessageRecipients`, and `ProjectsAgentPromptPage` — fixing the pill path brings mention autocomplete to parity, not beyond it.

### Fix

PR incoming from this account: admit mention candidates through the mentionable set (`getMentionableAgentPubkeys` = managed ∪ directory-eligible) at the `useMentions` call site, leaving `isAgentIdentityInManagedList` unchanged for its other consumer; unit tests included. Verified on a live self-hosted deployment (desktop vs Android discrepancy reproduced, then resolved with the patch).