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/modernc.org/mathutil/rat.go

@@ -0,0 +1,26 @@
+// Copyright (c) 2014 The mathutil Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package mathutil // import "modernc.org/mathutil"
+
+// QCmpUint32 compares a/b and c/d and returns:
+//
+//	-1 if a/b <  c/d
+//	 0 if a/b == c/d
+//	+1 if a/b >  c/d
+func QCmpUint32(a, b, c, d uint32) int {
+	switch x, y := uint64(a)*uint64(d), uint64(b)*uint64(c); {
+	case x < y:
+		return -1
+	case x == y:
+		return 0
+	default: // x > y
+		return 1
+	}
+}
+
+// QScaleUint32 returns a such that a/b >= c/d.
+func QScaleUint32(b, c, d uint32) (a uint64) {
+	return 1 + (uint64(b)*uint64(c))/uint64(d)
+}