Channel mention from a second owner device still mints a duplicate agent identity (desktop-v0.5.7, post-#4913)

[Open-luci]

Summary

With both devices of the same owner on desktop-v0.5.7, mentioning a managed agent from a second device in a channel (not a DM) still mints a second identity for that agent instead of reaching the running instance on the host device.

Steps to reproduce

1. On machine A, create a managed agent ("Test agent") and start it. Public key b060393e…4794.
2. On machine B (same owner, same workspace, also desktop-v0.5.7), open a channel both are in.
3. Mention @Test agent.

Expected

The mention reaches the running instance on machine A. No new identity is created.

Actual

Machine B mints a second identity for the same agent name, public key 2261ce75…ec44. Both are shown as "managed by you". The instance on machine A does not respond. Machine B renders a "needs configuration" card for the local instance it just created.

Notes

• fix(desktop): allow shared agent mentions #4913 ("allow shared agent mentions") appears to have addressed picker eligibility, but the sender-side spawn path (useMentionSendFlow → createMentionedPersonaAgents with ensureRunning: true) still instantiates locally.
• Reproduced in a channel, so this is not the deliberate DM fail-closed behaviour.
• Removing the local ACP adapter on machine B changes the symptom to an honest "adapter isn't installed" card, but does not prevent the mint attempt.
• Related: Managed-agent instance config (kind:30177) never syncs cross-device, causing every member device to auto-offer a duplicate/unconfigured local agent #3753, Mention picker re-mints a persona on a second device: inbound kind:30177 persona_id link is discarded #3414, Persona mentions from a second desktop fail to send and mint orphan agent keypairs #4634, [Bug] Second Desktop install creates duplicate agent identities, inflating channel membership #2648, Mentioning an agent from another machine never wakes it — agent start on mention is sender-side only #4658, Managed agents are only mentionable from the machine that runs them #3277.

Environment

macOS, desktop-v0.5.7 on both devices, self-hosted relay, single owner.