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

flake.nix

@@ -294,6 +294,7 @@
           "custom-tea-rbw" = pkgs.custom.tea-rbw;
           "custom-rclone-torbox-setup" = pkgs.custom.rclone-torbox-setup;
           "custom-opencode" = pkgs.custom.opencode;
+          "custom-qmd" = pkgs.custom.qmd;
 
           "qt-pinned-jellyfin-media-player" = pkgsQt.jellyfin-media-player;
           "qt-pinned-stremio" = pkgsQt.stremio;