Desktop: shared relay agents owned by other members are un-mentionable and un-addable (managed-list gate precedes directory eligibility)

## Problem

The desktop drops any agent-flagged identity that is not in the **local** managed-agents list before the relay-directory eligibility logic ever runs:

- `useMentions.ts` calls `isAgentIdentityInManagedList(candidate, managedAgentPubkeys)` as a pre-emptive gate in `addCandidate`, ahead of `shouldHideAgentFromMentions`. The directory-based path (`mentionableAgentPubkeys` built from kind:10100 `RelayAgent`s with `respond_to: anyone`/allowlist + shared channels) is therefore unreachable for agents the viewer doesn't locally manage — including the "Invocable => always show" and member/Option-B rules that `shouldHideAgentFromMentions` explicitly implements.
- `MembersSidebar.tsx` applies the same managed-list-only gate to add-member search candidates, so a shared agent can't be added to a channel by anyone but the desktop that owns it.

Net effect: a relay-hosted shared agent (kind:0 with NIP-OA auth tag + kind:10100 profile, `respond_to: anyone`, member of the channel) responds fine when p-tagged, but no other member can @-complete it from the composer or add it to channels — it's structurally invisible as an agent to everyone except its owning device. (Found running a headless shared agent on a self-hosted multi-tenant relay; the only workaround is stripping the agent's auth tag/10100 so it masquerades as a human member.)

## Expected

`shouldHideAgentFromMentions` is already the correct, complete policy (invocable → show; non-member non-invocable → hide; member + directory-excluded → hide; member + unknown → show). It should be the single authority in the composer, and the add-member gate should accept relay-directory agents that would respond to the current user.

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