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/inconshreveable/mousetrap/trap_others.go

@@ -0,0 +1,16 @@
+//go:build !windows
+// +build !windows
+
+package mousetrap
+
+// StartedByExplorer returns true if the program was invoked by the user
+// double-clicking on the executable from explorer.exe
+//
+// It is conservative and returns false if any of the internal calls fail.
+// It does not guarantee that the program was run from a terminal. It only can tell you
+// whether it was launched from explorer.exe
+//
+// On non-Windows platforms, it always returns false.
+func StartedByExplorer() bool {
+	return false
+}