feat(vscode): add resizable sidebar to agent manager (#509)

* feat(vscode): add resizable sidebar to agent manager

Add drag-to-resize support for the agent manager left sidebar using
the existing ResizeHandle component. Width defaults to 260px, can be
resized between 200px and 40% of the viewport, and persists across
webview state recoveries.

* fix(vscode): read window.innerWidth at drag time for accurate max sidebar width

Evaluate the viewport-based max constraint inside the onResize callback
so it reflects the current panel size, not the size at initial render.