feat: add multi-backend & V2 chat support, telemetry, model capability overrides ✨

**✨ Features**
- Add multi-backend aggregation and multi-repo commit generation so multiple LiteLLM proxies and multi-repo workspaces are supported.
- Introduce experimental V2 Chat Provider, telemetry & observability, model capability overrides, and advanced token counting to improve provider features and diagnostics.
- Add new configuration settings (backends, baseUrl, modelOverrides, modelCapabilitiesOverrides, emitUsageData, etc.) to expose the new features and behaviors.

**📝 Docs**
- Replace VS Code Marketplace badges with Open VSX badges and update README(s) to require VS Code 1.110+ and reflect new features and settings.
- Replace legacy settings docs with a structured settings table and update commands, license, and feedback/contribution sections for clarity and accuracy.
- Tweak wording/formatting (e.g., apostrophes, deprecation notes) across README files for consistency.

**🧪 Tests**
- Update integration error handling tests to stub provider._configManager.isConfigured and getConfig, supplying a local proxy URL and inactivityTimeout for test scenarios.