feat(Sky): Implement Tauri event bridge for sky:// channel communication

Add SkyBridge.ts - a new TypeScript module that subscribes to all `sky://` Tauri events emitted by Mountain and routes them to the VS Code workbench APIs or direct DOM manipulation.

The bridge implements handlers for 8 event categories:
- Editor: openDocument, saveAll, applyEdits
- Output: create/append/clear/show/dispose channels
- StatusBar: create/update/dispose items
- Progress: start/update/complete notifications
- Terminal: show/hide/resize
- Webview: message/dispose panels
- Native: openExternal URL
- UI: show-message-request, show-input-box-request, show-quick-pick-request

Mountain.astro now initializes the bridge after VS Code loads, exposing a `__CEL_WORKBENCH__` global proxy that provides access to VS Code's IWorkbench commands registry, environment APIs, and logger for SkyBridge to use.

This establishes the frontend half of the IPC channel between Mountain (Rust/Tauri) and Sky (TypeScript/Astro), enabling the backend to drive UI updates via Tauri events.

Author: NikolaRHristov