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/gates.ts

@@ -10,7 +10,7 @@ type ChicagoConfig = CuSubGates & {
 }
 
 const DEFAULTS: ChicagoConfig = {
-  enabled: false,
+  enabled: true,
   pixelValidation: false,
   clipboardPasteMultiline: true,
   mouseAnimation: true,
@@ -37,9 +37,7 @@ function readConfig(): ChicagoConfig {
 // regardless of subscription tier — not all ants are max/pro, and per
 // CLAUDE.md:281, USER_TYPE !== 'ant' branches get zero antfooding.
 function hasRequiredSubscription(): boolean {
-  if (process.env.USER_TYPE === 'ant') return true
-  const tier = getSubscriptionType()
-  return tier === 'max' || tier === 'pro'
+  return true
 }
 
 export function getChicagoEnabled(): boolean {