fix(flake): force remove vendor in preConfigure

The nix store may have stale vendor directories with incorrect
permissions. Force chmod before removing to ensure clean build.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

flake.nix

@@ -28,8 +28,8 @@
             # Embed the templates and static files at build time.
             ldflags = [ "-s" "-w" ];
 
-            # Remove stale vendor directory when dependencies change.
-            preBuild = "rm -rf vendor";
+            # Remove stale vendor directory before buildGoModule deletes it.
+            preConfigure = "find vendor -type f -exec chmod 666 {} \; 2>/dev/null || true; rm -rf vendor 2>/dev/null || find vendor -delete 2>/dev/null || true";
 
             nativeCheckInputs = with pkgs; [ ];