refactor(capture): screen capture in subprocess

- Move screen capture to child process
- Isolate GDI/BitBlt crashes from agent
- Use -capture flag for internal helper mode
- Add protocol for frame request/response
- Make monitor enumeration thread-safe

lib/instance/console_windows.go

@@ -14,6 +14,7 @@ var (
 	procSetConsoleCtrl   = kernel32.NewProc("SetConsoleCtrlHandler")
 	procAllocConsole     = kernel32.NewProc("AllocConsole")
 	procGetStdHandle     = kernel32.NewProc("GetStdHandle")
+	procGetFileType      = kernel32.NewProc("GetFileType")
 	user32               = syscall.NewLazyDLL("user32.dll")
 	procShowWindow       = user32.NewProc("ShowWindow")
 	consoleCtrlCallback  uintptr
@@ -81,6 +82,9 @@ func consoleCtrlHandler(ctrlType uintptr) uintptr {
 }
 
 func redirectStdioToNul() {
+	if stdioIsPipe() {
+		return
+	}
 	nul, err := os.OpenFile("NUL", os.O_RDWR, 0)
 	if err != nil {
 		return
@@ -89,3 +93,12 @@ func redirectStdioToNul() {
 	os.Stdout = nul
 	os.Stderr = nul
 }
+
+func stdioIsPipe() bool {
+	const fileTypePipe = 3
+	stdin, _, _ := procGetStdHandle.Call(^uintptr(9))
+	stdout, _, _ := procGetStdHandle.Call(^uintptr(10))
+	inType, _, _ := procGetFileType.Call(stdin)
+	outType, _, _ := procGetFileType.Call(stdout)
+	return inType == fileTypePipe || outType == fileTypePipe
+}