Linked built-in agents discard provider/model edits and return to “needs configuration”

**Describe the bug**

Provider and model selections made through **Edit Agent** are not persisted for linked built-in agents such as Honey and Bumble. After restarting Buzz, the agents return to “needs configuration” and ask for both fields again.

**Steps to reproduce**

1. Add or start a built-in agent such as Honey or Bumble using `buzz-agent`.
2. Open **Edit Agent**.
3. Select a provider and model.
4. Save the changes.
5. Restart Buzz or restart the agent.
6. Send the agent a message.

The agent reports that its provider and model are not configured.

**Expected behavior**

The provider and model selected in **Edit Agent** should remain effective after saving and restarting.

**Version and platform**

- Buzz version: 0.5.3
- OS: macOS 26.5.1

**Logs / additional context**

The linked agent instances resolve provider and model from their persona definitions, but the edit dialog omits both fields from its update whenever `linkedPersona != null`:

https://github.com/block/buzz/blob/desktop-v0.5.3/desktop/src/features/agents/ui/AgentInstanceEditDialog.tsx#L681-L710

Effective configuration then reads those fields from the linked definition:

https://github.com/block/buzz/blob/desktop-v0.5.3/desktop/src-tauri/src/managed_agents/effective_config/mod.rs#L74-L96

Because the built-in definitions have no provider or model defaults, the visible selections become a no-op. The dialog should either update the linked definition or direct the user to a definition-level configuration surface.