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/fcntl/capi_windows_arm64.go

@@ -0,0 +1,5 @@
+// Code generated by 'ccgo fcntl\gen.c -crt-import-path  -export-defines  -export-enums  -export-externs X -export-fields F -export-structs  -export-typedefs  -header -hide _OSSwapInt16,_OSSwapInt32,_OSSwapInt64 -o fcntl\fcntl_windows_arm64.go -pkgname fcntl', DO NOT EDIT.
+
+package fcntl
+
+var CAPI = map[string]struct{}{}