Provider-backed agent delete is routed through persona deletion and fails

[nedos]

Describe the bug

Deleting a provider-backed agent from the Agents card opens the persona deletion flow. The dialog says it will also delete the linked agent instance, but confirmation fails with:

persona <id> has provider-deployed agent instances (<name>); delete those agent instances first

Steps to reproduce

1. Create an agent using an external provider.
2. Open the agent card’s ⋮ menu.
3. Select Delete and confirm.

Expected behavior

The action deletes the provider-backed agent instance, or directs the user to the instance deletion action.

Actual behavior

It enters the persona cascade-delete path, promises to delete the instance, then refuses because the instance is provider-deployed.

Possibly related: #2798.

Version and platform

• Buzz: 0.5.2
• macOS: 26.5.2