Mention picker still leaks same-owner agents from other communities after #6086

# Mention picker still leaks same-owner agents from other communities after #6086

## Summary

Buzz Desktop 0.5.18 still offers locally managed agents from other communities
in the `@` mention picker, even when those pubkeys are not members of the
current channel. This is a residual/regression of #3204 after merged PR #6086.

The same setup also exposes the shared-persona naming problem from #5128 and
#5705: separate community-specific Fizz identities share `persona_id:
builtin:fizz`, so a persona rename propagates across communities while a stale
instance can keep the old name.

## Environment

- Buzz Desktop 0.5.18 (`desktop-v0.5.18`, release commit `39f8b46`)
- macOS
- Communities configured in one Desktop: `gtm-graph`, `hands-on`, `tococare`,
`artisan-os`
- Built-in Welcome Team agents

## Reproduction

1. Open a DM in `gtm-graph` whose relay member list contains exactly the human
and one Fizz identity.
2. Open the thread composer and click the mention button.
3. Observe the candidate list.

## Expected

The picker offers the two channel members, including exactly one Fizz identity.

## Actual

The picker offers 13 candidates, including multiple Fizz, Honey, and Bumble
identities from other communities. It includes both `Fizz` and several
`Fizz GTM` entries.

The verified channel roster contains only:

- current Fizz: `56c35b5d…`
- human owner: `a4c3d674…`

Sanitized local registry evidence:

| Community | Fizz public key | Name | Persona |
|---|---|---|---|
| gtm-graph | `56c35b5d…` | Fizz GTM | `builtin:fizz` |
| hands-on | `303b5d066…` | Fizz GTM | `builtin:fizz` |
| tococare | `2eec28f0d…` | Fizz GTM | `builtin:fizz` |
| artisan-os | `010e70557…` | Fizz | `builtin:fizz` |

There is also one definition record with no pubkey whose display name is
`Fizz GTM` and whose ID is `builtin:fizz`.

## Why this looks like a residual/regression

- #3204 reported that the picker used locally managed agents rather than the
current channel roster.
- #3204 was closed by #6086, merged on 2026-08-17.
- Desktop 0.5.18 was released on 2026-08-21 and still reproduces the
same-owner, cross-community case.

## Related

- #3204 — Mention picker lists locally-managed agents instead of channel members
- #6086 — enforce shared agent access across devices
- #5128 — Agents tab is not community-scoped
- #5705 — Persona rename leaves linked instances on the old relay identity

## Privacy note

The raw `managed-agents.json` must not be attached because it contains private
runtime and authentication material. The table above is the sanitized evidence.