Add Nix flake for mcp and mcp-agent

Exposes two packages:
- default (mcp CLI) for operator workstations
- mcp-agent for managed nodes

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

vendor/github.com/mattn/go-isatty/isatty_plan9.go

@@ -0,0 +1,23 @@
+//go:build plan9
+// +build plan9
+
+package isatty
+
+import (
+	"syscall"
+)
+
+// IsTerminal returns true if the given file descriptor is a terminal.
+func IsTerminal(fd uintptr) bool {
+	path, err := syscall.Fd2path(int(fd))
+	if err != nil {
+		return false
+	}
+	return path == "/dev/cons" || path == "/mnt/term/dev/cons"
+}
+
+// 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
+}