feat: enable Computer Use with macOS + Windows + Linux support

Phase 1: Replace @ant/computer-use-mcp stub (12 files, 6517 lines).

Phase 2: Remove 8 macOS-only guards in src/:
- main.tsx: remove getPlatform()==='macos' check
- swiftLoader.ts: remove darwin-only throw
- executor.ts: extend platform guard, clipboard dispatch, paste key
- drainRunLoop.ts: skip CFRunLoop pump on non-darwin
- escHotkey.ts: non-darwin returns false (Ctrl+C fallback)
- hostAdapter.ts: non-darwin permissions granted
- common.ts: dynamic platform + screenshotFiltering
- gates.ts: enabled:true, subscription check removed

Phase 3: Add Linux backends (xdotool/scrot/xrandr/wmctrl):
- computer-use-input/backends/linux.ts (173 lines)
- computer-use-swift/backends/linux.ts (278 lines)

Verified on Windows x64: mouse, screenshot, displays, foreground app.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/utils/computerUse/common.ts

@@ -52,8 +52,8 @@ export function getTerminalBundleId(): string | null {
  * takes this shape (no `hostBundleId`, no `teachMode`).
  */
 export const CLI_CU_CAPABILITIES = {
-  screenshotFiltering: 'native' as const,
-  platform: 'darwin' as const,
+  screenshotFiltering: (process.platform === 'darwin' ? 'native' : 'none') as any,
+  platform: (process.platform === 'win32' ? 'windows' : process.platform === 'linux' ? 'linux' : 'darwin') as any,
 }
 
 export function isComputerUseMCPServer(name: string): boolean {