Desktop @-mention autocomplete only shows locally managed agents

## Purpose

Teams need to `@mention` shared / org-level agents from their own Buzz Desktop — for example a research agent owned by someone else but configured with `respond_to: allowlist` or `anyone` and present in a shared channel.

Today that is impossible through the UI. Non-owners only see agents managed by **their own Desktop install**. `respond_to` settings cannot fix it.

## Blocker

In Desktop mention autocomplete, agent candidates are filtered by `isAgentIdentityInManagedList` **before** the `respond_to` / shared-channel eligibility logic runs.

`managedAgentPubkeys` comes from `listManagedAgents` (local Tauri command for this install). Any channel member with `role: "bot"` whose pubkey is not in that local set is dropped.

So the allowlist / anyone eligibility code in `relayAgentIsSharedWithUser` / `shouldHideAgentFromMentions` is unreachable for cross-owner agents.

## Actual vs expected

| | |
|---|---|
| **Actual** | `@` list shows only agents this Desktop manages. Cross-owner channel bots never appear. |
| **Expected** | Agents that are channel members and that the viewer is allowed to invoke (`respond_to: anyone` + shared channel, or `respond_to: allowlist` including the viewer) appear in `@` autocomplete. |

## Repro

1. User A owns agent X; agent X is a member of a shared channel; `respond_to` is `allowlist` (includes User B) or `anyone`.
2. User B opens that channel on their own Desktop (does not manage agent X).
3. User B types `@` in the composer.

**Result:** Agent X is missing. User B only sees agents their install manages.

**Note:** Explicit CLI `--mention ` still delivers (relay accepts it). This is a Desktop autocomplete UI gate, not a relay delivery failure.

## Code

- `desktop/src/features/messages/lib/useMentions.ts` (~244–262) — `addCandidate` order: managed-list filter before eligibility hide
- `desktop/src/features/agents/lib/agentAutocompleteEligibility.ts` (`isAgentIdentityInManagedList`, `relayAgentIsSharedWithUser`)
- `desktop/src/features/agents/hooks.ts` (`useManagedAgentsQuery` → local `listManagedAgents`)

## Out of scope for this issue

Whether the harness author gate wakes the agent after a successful mention. That is a separate path. This issue is only: **non-owners cannot select the agent in Desktop `@` autocomplete.**

Start in desktop/src/features/messages/lib/useMentions.ts around addCandidate and trace the eligibility helpers in desktop/src/features/agents/lib/agentAutocompleteEligibility.ts, including the managed-agent query in hooks.ts. Reproduce with a non-owner viewing a shared-channel agent, then verify that allowed anyone/allowlist agents appear while disallowed agents and existing local filtering behavior remain correct.

Written by the indexing model from the issue text.