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

## 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

- #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: #3753, #3414, #4634, #2648, #4658, #3277.

## Environment

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

Trace the sender-side useMentionSendFlow → createMentionedPersonaAgents path for channel mentions, focusing on the ensureRunning: true behavior described in the issue. Reproduce the two-device channel scenario, then verify that the mention reaches the existing host instance without minting a local identity or showing a local configuration card.

Written by the indexing model from the issue text.