🚀 Release v2.1.6 — VS Code picker crash workaround (#105)

* fix: 🐛 prevent VS Code picker crashes from non-string model categories

**🐛 Fixes**
- Add `derivePickerCategory()` to map model capabilities to VS Code's supported `lightweight`, `versatile`, or `powerful` tags. This avoids passing an object into the picker and prevents the `a.charAt is not a function` crash.
- Update the LiteLLM provider registry to emit a string `category` for selectable models. Keep grouping behavior intact by relying on the picker’s existing vendor/group lookup instead of `category`.

**🧪 Tests**
- Extend model capability tests to cover all picker category branches, including the `undefined` fallback. Add regression checks to ensure the helper never returns a non-string value.
- Update provider display tests to assert on the fields the picker actually reads and add coverage for balanced, reasoning, lightweight, and filtered-out fallback models. This protects against future picker regressions.

**🧹 Chores**
- Add an `npm run check` script that chains lint, format, and coverage tests. Bump the package version to `2.1.6`.
- Remove the unused DeepWiki MCP server from the VS Code config and refresh the investigative agent tool list. This keeps workspace metadata aligned with the current setup.

* 📝 Add changelog entry for v2.1.6 release

- Document the picker-crash workaround (bug #104)
- List the new derivePickerCategory tests
- Note the npm run check script addition and version bump