Cannot delete an agent when its managed-agent dependency is not visible or reassignable

**Describe the bug**
Deleting an agent can fail because it is “still assigned to a managed agent,” but the Desktop UI does not identify the dependent agent or provide a way to reassign that relationship.

**Steps to reproduce**
1. Open **Agents** in Buzz Desktop.
2. Open the actions menu for an agent (in this case, **Olda Boty**).
3. Select **Delete**.
4. Observe the error.

**Expected behavior**
The UI should either:
- identify every managed agent that depends on the agent being deleted and link to it; or
- provide a supported reassignment/removal path before deletion.

**Actual behavior**
A toast appears:

`Olda Boty is still assigned to a managed agent. Remove or reassign those agents first.`

The current agent editor exposes only a read-only **Managed by** link. There is no editable manager/reassignment control, and no visible list of the dependent managed agents.

**Version and platform**
- Buzz version: Version 0.5.23 (0.5.23)
- OS: macOS 26.6.2 (25G83)
**Logs / additional context**
This prevents removal of an unused agent and leaves the user without a discoverable remediation path.

Start in Buzz Desktop's Agents view by tracing the actions-menu delete flow and the agent editor's read-only Managed by link. Reproduce the Olda Boty case and inspect how the dependency error is surfaced. Done means dependent managed agents are discoverable and a supported reassignment or removal path allows deletion.

Written by the indexing model from the issue text.