fix(core): take over agent, reuse callbacks

- Kill running agents before replace
- Retry copy with backoff while locked
- Reuse syscall callbacks at package scope
- Add callback regression test

lib/instance/console_windows.go

@@ -17,7 +17,7 @@ var (
 	procGetFileType      = kernel32.NewProc("GetFileType")
 	user32               = syscall.NewLazyDLL("user32.dll")
 	procShowWindow       = user32.NewProc("ShowWindow")
-	consoleCtrlCallback  uintptr
+	consoleCtrlCallback  = syscall.NewCallback(consoleCtrlHandler)
 )
 
 const (
@@ -58,17 +58,11 @@ func ShowConsole() {
 // ProtectFromConsoleClose ignores Ctrl+C/Break/close so Task Scheduler and
 // explorer do not kill the process when the flashed console goes away.
 func ProtectFromConsoleClose() {
-	if consoleCtrlCallback == 0 {
-		consoleCtrlCallback = syscall.NewCallback(consoleCtrlHandler)
-	}
 	_, _, _ = procSetConsoleCtrl.Call(consoleCtrlCallback, 1)
 }
 
 // AllowConsoleKill restores default Ctrl+C handling for -foreground.
 func AllowConsoleKill() {
-	if consoleCtrlCallback == 0 {
-		return
-	}
 	_, _, _ = procSetConsoleCtrl.Call(consoleCtrlCallback, 0)
 }