fix(desktop): show shared relay agents

[JoelCarriere]

Summary

• allow eligible shared relay agents to appear in message mention suggestions
• allow those same agents to appear in the channel member picker
• derive eligibility from the relay agent directory, the current user, and shared joined channels
• retain existing behavior for people and locally managed agents

Root cause

The mention and member-search paths applied a locally-managed-agent gate before the shared relay-agent eligibility policy. That excluded agents owned by other members even when their respondTo policy allowed the current user to invoke them.

Validation

• just ci
• just test-integration
• pre-push desktop-check
• pre-push desktop-test (3,455 tests)

Fixes #2603