feat: add QMD v2.1.0 as Nix package, bake into openclaw image

- packages/qmd: buildNpmPackage with Node.js 22 (not Bun) to avoid
  native module ABI issues with better-sqlite3 and sqlite-vec
- Vendored package-lock.json (QMD ships bun.lock, not npm lockfile)
- packages/openclaw-image: adds qmd + tsx to image contents
- packages/default.nix: rec attrset so openclaw-image can inherit qmd
- flake.nix: expose custom-qmd package output for CI caching

packages/default.nix

@@ -1,5 +1,5 @@
 { pkgs, ... }:
-{
+rec {
   tea-rbw = pkgs.callPackage ./tea-rbw { };
   app-launcher-server = pkgs.callPackage ./app-launcher-server { };
   claude-code = pkgs.callPackage ./claude-code { };
@@ -8,5 +8,6 @@
   pi-coding-agent = pkgs.callPackage ./pi-coding-agent { };
   nextcloud-talk-desktop = pkgs.callPackage ./nextcloud-talk-desktop { };
   opencode = pkgs.callPackage ./opencode { };
-  openclaw-image = pkgs.callPackage ./openclaw-image { };
+  qmd = pkgs.callPackage ./qmd { };
+  openclaw-image = pkgs.callPackage ./openclaw-image { inherit qmd; };
 }