Include assets/chrome-extension in gateway package

[truongsinh]

Summary

• Copy the assets/ directory to the gateway output alongside extensions/ during install
• Add package-contents check for assets/chrome-extension/manifest.json

Problem

openclaw browser extension install fails with:

Error: Bundled Chrome extension is missing. Reinstall OpenClaw and try again.

The runtime code (resolveBundledExtensionRootDir in browser-cli-extension.ts) walks up from dist/ looking for assets/chrome-extension/manifest.json. The assets/ directory exists in the source tree but gateway-install.sh doesn't copy it to $out/lib/openclaw/.

Workaround

Users can work around this with an overlay:

(final: prev: let
  openclaw-gateway-patched = prev.openclaw-gateway.overrideAttrs (old: {
    installPhase = ''
      ${old.installPhase}
      if [ -d "$src/assets" ]; then
        cp -r "$src/assets" "$out/lib/openclaw/"
      fi
    '';
  });
in {
  openclaw-gateway = openclaw-gateway-patched;
  openclaw = prev.openclaw.override { openclaw-gateway = openclaw-gateway-patched; };
})

Test plan

• nix build .#openclaw-gateway succeeds
• nix flake check passes (including package-contents check)
• openclaw browser extension install works with the built package