[Bug] Agents created outside the current app-managed store do not appear in the Agents tab

## Summary

Agents created through the external `buzz-acp` CLI, or through a different Buzz app bundle/profile, do not appear in the desktop app's Agents tab.

This makes agents that are present and active on the local relay look as if they were missing.

## Observed behavior

- The local relay contains the agent and the agent can participate in channels/respond through `buzz-acp`.
- The desktop app's Agents tab does not list that agent.
- The tab only shows agents registered in the current app's local managed-agent store.
- The production and development app bundles use separate managed-agent stores, so agents created in one are not automatically visible in the other.

## Expected behavior

The Agents tab should either:

1. discover and display agents associated with the current community/relay, including externally managed `buzz-acp` agents, or
2. clearly distinguish app-managed agents from externally managed agents and provide an explicit import/adopt flow.

If adopting an existing agent is supported, it should preserve the agent's existing identity where possible rather than silently creating a duplicate.

## Steps to reproduce

1. Start a local Buzz relay.
2. Create/configure an agent using the external `buzz-acp` CLI and connect it to the local relay.
3. Confirm that the agent is present in a channel and can respond to a message.
4. Open the Buzz desktop app connected to the same local relay/community.
5. Open the Agents tab.
6. Observe that the externally managed agent is not listed.

The same issue can be reproduced by creating an app-managed agent in a development build and then opening the production build.

## Environment

- Buzz Desktop: 0.4.26
- Local self-hosted relay
- External agent runtime: `buzz-acp`
- macOS

## Notes

The current behavior appears to come from the Agents view reading only the current app bundle's local managed-agent storage rather than merging relay-visible agents or offering an import path.

Start with the desktop app's Agents tab and the current app bundle's local managed-agent storage described in the report. Reproduce the issue with an external buzz-acp agent and with agents created across development and production bundles, then compare those results with agents visible on the local relay. Done means externally managed or cross-bundle agents are discoverable, or are clearly distinguished and can be adopted without silently creating duplicates.

Written by the indexing model from the issue text.