fix(desktop): mention existing agents across devices

[timigod]

Summary

• allow a client with no local agent runtimes to mention an existing eligible relay agent
• preserve the existing managed-persona path when a genuinely local managed instance exists
• cover the cross-device DM case with a focused Playwright regression

Why

A chat-only desktop can already see the canonical agent identity running on another device, but mention autocomplete currently filters that identity out and offers the same-named local persona instead. Selecting that persona tries to create a local runtime and ends in No agent runtime available.

This keeps the identity on the relay authoritative for chat. Mentioning an existing remote agent does not create or start a local managed agent.

Verification

• desktop TypeScript typecheck
• Biome/check scripts for the affected client files
• all 5,556 desktop unit tests in the combined patch tree
• focused Playwright: an existing remote agent in a DM is mentioned without creating a local runtime