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/hostAdapter.ts

@@ -45,6 +45,7 @@ export function getComputerUseHostAdapter(): ComputerUseHostAdapter {
       getHideBeforeActionEnabled: () => getChicagoSubGates().hideBeforeAction,
     }),
     ensureOsPermissions: async () => {
+      if (process.platform !== 'darwin') return { granted: true }
       const cu = requireComputerUseSwift()
       const accessibility = (cu as any).tcc.checkAccessibility()
       const screenRecording = (cu as any).tcc.checkScreenRecording()