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_solaris.go

@@ -0,0 +1,21 @@
+//go:build solaris && !appengine
+// +build solaris,!appengine
+
+package isatty
+
+import (
+	"golang.org/x/sys/unix"
+)
+
+// IsTerminal returns true if the given file descriptor is a terminal.
+// see: https://src.illumos.org/source/xref/illumos-gate/usr/src/lib/libc/port/gen/isatty.c
+func IsTerminal(fd uintptr) bool {
+	_, err := unix.IoctlGetTermio(int(fd), unix.TCGETA)
+	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
+}