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>

packages/@ant/computer-use-mcp/src/subGates.ts

@@ -0,0 +1,19 @@
+import type { CuSubGates } from './types.js'
+
+export const ALL_SUB_GATES_ON: CuSubGates = {
+  pixelValidation: true,
+  clipboardPasteMultiline: true,
+  mouseAnimation: true,
+  hideBeforeAction: true,
+  autoTargetDisplay: true,
+  clipboardGuard: true,
+}
+
+export const ALL_SUB_GATES_OFF: CuSubGates = {
+  pixelValidation: false,
+  clipboardPasteMultiline: false,
+  mouseAnimation: false,
+  hideBeforeAction: false,
+  autoTargetDisplay: false,
+  clipboardGuard: false,
+}