Mention picker validates against local managed-agents store instead of relay membership (kind:39002); locally registering remote agents intercepts mentions

## Summary
Two related problems for agents that are relay members but run on a remote host (external buzz-acp instances, not spawned by the desktop app):

1. **False "not in channel" warning.** The @-mention picker warns that these agents are not in the channel even though the channel's kind:39002 member event lists their pubkeys and they respond to mentions normally. The picker appears to validate against the local managed-agents store rather than the relay's membership events (which the app already receives).

2. **Local registration intercepts mentions.** If such agents are added to the local store (`agents/managed-agents.json`) so they render in the Agents tab, mentioning them fails hard: "Could not start agent mention: : This agent's configuration is missing — it may still be syncing or was deleted on another device." The app tries to launch a local copy instead of just publishing the mention; the message never reaches the relay. There is no way to register an agent as display-only/remote.

## Repro
1. Self-hosted relay; agent runs buzz-acp on a separate Linux host, admitted via `buzz-admin add-member` + a row in `channel_members` + regenerated 39000/39002 (`reconcile-channels`).
2. Desktop app (Windows, v0.5.x): mention the agent → warning "not in channel", but delivery works and the agent replies.
3. Add the agent to `managed-agents.json` (valid entry, `is_active: false`, no local provider config) → mentioning now errors and nothing is published.

## Expected
- Picker membership check sourced from the channel's 39002 event (or channel_members), not the local spawn registry.
- Mentioning an agent the app cannot start locally should still publish the mention; local auto-start should be an optimization, never a gate.

## Environment
Desktop v0.5.x on Windows 11; self-hosted relay (compose stack); agents via buzz-acp + external harness on Ubuntu, systemd-managed.