chore: add weekly VS Code compatibility review prompt

**🧹 Chores**
- Add a reusable prompt for weekly VS Code compatibility reviews, covering the review window, upstream sources, required analysis, and report format.
- Document Brave Search MCP recovery steps in `AGENTS.md` so search requests retry cleanly after startup-related `invoke` errors.

**🧪 Tests**
- Strengthen the LiteLLM provider base test to assert `reasoningEffort` stays grouped under the navigation model-picker action.