Summary

• allow agents managed on another machine to appear in @ autocomplete when they are members of the current channel
• preserve the existing respond-to policy check and keep non-member remote agents hidden
• route the selected agent pubkey as a Nostr p tag without trying to start the remote process locally
• add unit and desktop E2E regression coverage

Root cause

The mention pipeline correctly identified shared relay agents as invocable, but then applied a local-managed-agent filter before the policy-aware eligibility check. That removed every agent not managed by the current desktop, including valid members running on another machine. Manually typing the name did not restore the missing pubkey tag, so the remote ACP process never received the mention.

User impact

Users connected to the same community can now discover and invoke an in-channel agent from another machine, subject to that agent's existing respond-to policy.

Manual test

1. Started an agent on Machine A and set Respond-to to Anyone.
2. Added the agent and the Machine B user to the same channel.
3. Ran this branch on Machine B with just desktop-standalone.
4. Confirmed the remote agent appeared in @ autocomplete and responded after selection.

Validation

• just ci
• focused unit regression test
• focused Playwright smoke tests for remote-agent visibility, Nostr p-tag routing, and no local process start
• no new unwrap() or unsafe code
• no public API, event kind, endpoint, or configuration documentation required
• just test not run; this is a desktop-only change and does not modify relay, database, or auth behavior

Fixes #2508