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_others.go

@@ -0,0 +1,17 @@
+//go:build (appengine || js || nacl || tinygo || wasm) && !windows
+// +build appengine js nacl tinygo wasm
+// +build !windows
+
+package isatty
+
+// IsTerminal returns true if the file descriptor is terminal which
+// is always false on js and appengine classic which is a sandboxed PaaS.
+func IsTerminal(fd uintptr) bool {
+	return false
+}
+
+// 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
+}