metamcp: package v2.4.22 as a NixOS-deployable service

Single-derivation pnpm/Turbo monorepo build producing:
  - metamcp           full orchestrator (waits for PG, runs drizzle
                      migrations, launches backend :12009 + frontend :12008)
  - metamcp-backend   bare backend launcher
  - metamcp-frontend  Next.js standalone server.js launcher

Notes:
  - Upstream pins packageManager: pnpm@9.0.0; rewrite to match the nixpkgs
    pnpm (Turbo requires the field but won't fight a matching version).
  - Frontend uses next.config 'output: standalone'; we copy .next/static
    and public/ into the standalone tree since Next doesn't.
  - HOSTNAME defaults to 0.0.0.0 (override with METAMCP_HOSTNAME) — Next
    standalone otherwise inherits the system hostname and is unreachable
    on 127.0.0.1.

flake.nix

@@ -57,6 +57,7 @@
         {
           hello-world = pkgs.callPackage ./packages/hello-world.nix { };
           geesefs = pkgs.callPackage ./packages/geesefs.nix { };
+          metamcp = pkgs.callPackage ./packages/metamcp.nix { };
           xonsh = pkgs.callPackage ./packages/xonsh.nix {
             xonsh-unwrapped = import ./packages/xonsh-unwrapped.nix {
               inherit (pkgs) lib python3Packages fetchFromGitHub;