Preserve the full upstream snapshot with symlink-aware world copies

flake.nix

@@ -104,8 +104,6 @@
           postPatch = ''
             git init --quiet --initial-branch=main
             git add host vfs userland extensions agents skills Cargo.toml Cargo.lock flake.nix flake.lock README.md AGENTS.md .gitignore
-            # This browser fixture links to an unbuilt upstream artifact, not a WASI tool.
-            git rm --cached userland/esbuild/source/scripts/browser/esbuild.wasm.bagel
             git commit --no-gpg-sign --quiet -m 'Bundle source ${revision}'
             cp -r ${brushSource} userland/brush/source
             chmod -R u+w userland/brush/source
@@ -138,6 +136,8 @@
             cp -a target/userland/bundle/. $out/
           '';
           doInstallCargoArtifacts = false;
+          # The upstream browser fixture links to an optional, unbuilt artifact.
+          dontCheckForBrokenSymlinks = true;
         });
         sausage = craneLib.buildPackage (common // {
           inherit cargoArtifacts;