Track Compute Share managed-agent startup, MeshLLM diagnostics, and relay-mesh text-only media handling

## Summary

Follow-up tracker for the broader Compute Share / managed-agent / relay-mesh behaviors validated while fixing Windows MeshLLM desktop builds in #3223.

The Windows packaging issue (#2836) uncovered and/or required fixes beyond simply enabling the `mesh-llm` feature for Windows installers:

- Managed `buzz-agent` should start correctly from the installed desktop app.
- Relay-mesh LLM calls should not fail with text-only model media-input `422` errors caused by replaying prior tool-result images into later requests.
- MeshLLM diagnostic logging should be available as an opt-in runtime toggle from Compute → Advanced.

## Validation target

A desktop install should be able to:

1. Start Compute Share.
2. Start a managed `buzz-agent` from the installed app.
3. Route relay-mesh text requests to shared compute without replayed image/tool-result history causing `422 Unprocessable Entity` on text-only models.
4. Enable/disable MeshLLM diagnostic logging from the Compute advanced settings without rebuilding or setting environment variables.

## Related

- PR: #3223
- Original Windows packaging issue: #2836