feat: add unity_screenshot_editor_window tool (v2.30.0)

Capture any Editor window (Inspector, Project, Console, custom windows)
  to a PNG. Unlike the camera-based scene/game capture, it grabs the real
  editor UI via the C# plugin's Win32 PrintWindow path — occlusion-proof.

  - New bridge method captureEditorWindow → route "screenshot/editor-window"
  - New core tool unity_screenshot_editor_window (args: window [required],
    path, maxDimension); default Assets/Screenshots/, any .png path allowed
  - Tool description gates usage: invoked only on an explicit user request,
    and returns/tells "unsupported platform" on macOS/Linux (Windows-only)
  - Registered in tool-tiers core list + manifest.json; README "What's New"