feat(Mountain): implement configuration management handlers

- Added `handlers/config.rs` with core functions for resolving, loading, merging, and persisting configuration files (settings.json) across User, Workspace, and Folder scopes
- Introduced JSON path-based updates to support granular configuration changes required by VS Code extension APIs
- Implemented Vine IPC notifications to Cocoon on config changes, maintaining extension host compatibility
- Integrated with AppState for workspace folder tracking and config path resolution
- Added async file I/O with proper error handling and directory creation
- Created merge logic prioritizing workspace/folder settings over user defaults per VS Code behavior

Enables persistent configuration storage and dynamic updates critical for extension functionality in the Cocoon shim layer. Provides foundation for language-specific settings and workspace-aware features.

Author: NikolaRHristov