Add Nix flake for mcproxyctl

Vendor dependencies and expose mcproxyctl binary via nix build.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

vendor/github.com/mattn/go-isatty/isatty_bsd.go

@@ -0,0 +1,20 @@
+//go:build (darwin || freebsd || openbsd || netbsd || dragonfly || hurd) && !appengine && !tinygo
+// +build darwin freebsd openbsd netbsd dragonfly hurd
+// +build !appengine
+// +build !tinygo
+
+package isatty
+
+import "golang.org/x/sys/unix"
+
+// IsTerminal return true if the file descriptor is terminal.
+func IsTerminal(fd uintptr) bool {
+	_, err := unix.IoctlGetTermios(int(fd), unix.TIOCGETA)
+	return err == nil
+}
+
+// IsCygwinTerminal return true if the file descriptor is a cygwin or msys2
+// terminal. This is also always false on this environment.
+func IsCygwinTerminal(fd uintptr) bool {
+	return false
+}