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/google.golang.org/protobuf/internal/impl/codec_unsafe.go

@@ -0,0 +1,15 @@
+// Copyright 2019 The Go 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 impl
+
+// When using unsafe pointers, we can just treat enum values as int32s.
+
+var (
+	coderEnumNoZero      = coderInt32NoZero
+	coderEnum            = coderInt32
+	coderEnumPtr         = coderInt32Ptr
+	coderEnumSlice       = coderInt32Slice
+	coderEnumPackedSlice = coderInt32PackedSlice
+)