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/libc/libc_arm64.go

@@ -0,0 +1,40 @@
+// Copyright 2023 The Libc Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build !(linux && arm64)
+
+package libc // import "modernc.org/libc"
+
+import (
+	"fmt"
+	"unsafe"
+)
+
+// Byte loads are atomic on this CPU.
+func a_load_8(addr uintptr) uint32 {
+	return uint32(*(*byte)(unsafe.Pointer(addr)))
+}
+
+// int16 loads are atomic on this CPU when properly aligned.
+func a_load_16(addr uintptr) uint32 {
+	if addr&1 != 0 {
+		panic(fmt.Errorf("unaligned atomic 16 bit access at %#0x", addr))
+	}
+
+	return uint32(*(*uint16)(unsafe.Pointer(addr)))
+}
+
+// Byte sores are atomic on this CPU.
+func a_store_8(addr uintptr, b byte) {
+	*(*byte)(unsafe.Pointer(addr)) = b
+}
+
+// int16 stores are atomic on this CPU when properly aligned.
+func a_store_16(addr uintptr, n uint16) {
+	if addr&1 != 0 {
+		panic(fmt.Errorf("unaligned atomic 16 bit access at %#0x", addr))
+	}
+
+	*(*uint16)(unsafe.Pointer(addr)) = n
+}