fix(desktop): mention live relay agents instead of re-instantiating personas

[dilate7]

Summary

Persona mentions sent from a desktop that is not the agent's host machine try to spawn a local instance and fail (harness definitions are per-machine), blocking the send and minting a fresh orphan keypair per attempt.

This change makes createMentionedPersonaAgents resolve a persona mention to an already-live relay agent with the same display name when one exists, preferring an agent that is already a member of the current channel. Instantiation from the persona still happens when no live agent exists, so the summon-into-channel flow is unchanged on the host machine.

Not included: hiding persona autocomplete candidates that duplicate a live relay agent. That would touch useMentions.ts, which sits at the file-size ratchet cap — happy to do it as a follow-up with the extraction that requires, if wanted.

Related issue

Fixes #4634

Testing

• pnpm -C desktop typecheck, biome check, and the mention test suites (useMentions.test.mjs, mentionCandidates.test.mjs) pass.
• The failure was reproduced on the two-desktop setup described in the issue (macOS dev-build host + Windows v0.5.4 client on a self-hosted relay): channel mentions from the non-host desktop fail with the harness error while DMs work.
• End-to-end verification of the fix on the non-host desktop is pending a patched Windows build (in progress); will report back on this PR.

[dilate7]

Withdrawing this for now — verification on a second desktop surfaced an additional spawn path (managed-agent start on mention) that this change doesn't fully cover yet. I'll follow up once the fix is verified end-to-end. The issue (#4634) still stands and reproduces as described.