fix: change CWD to /tmp in _ensure_yarn_installed

All yarn/npm commands now run from /tmp to completely avoid workspace
package.json with patch: entries. Restores original CWD after install.

npm_install.py

@@ -2142,6 +2142,10 @@ def _build_frontend(src_frontend: Path, dest_frontend: Path):
             step_msg = f"Reinstalling Yarn 4.x (attempt {retry_count + 1}/{max_retries + 1})"
         
         with step(step_msg):
+            # Change CWD to /tmp to avoid workspace/package.json issues
+            original_cwd = os.getcwd()
+            os.chdir("/tmp")
+            
             # Cleanup on retry
             if retry_count > 0:
                 _cleanup_yarn_artifacts()
@@ -2202,6 +2206,7 @@ def _build_frontend(src_frontend: Path, dest_frontend: Path):
                         if major and major >= 4:
                             if DEBUG:
                                 print(f"   ✓ Yarn {major}.x installed via corepack")
+                            os.chdir(original_cwd)
                             return True
                 except Exception as e:
                     if DEBUG:
@@ -2245,7 +2250,8 @@ def _build_frontend(src_frontend: Path, dest_frontend: Path):
                     
                     if major and major >= 4:
                         if DEBUG:
-                            print(f"   ✓ Yarn {major}.x installed via @yarnpkg/cli")
+                            print(f"   ✓ Yarn {major}.x installed via 'yarn set version'")
+                        os.chdir(original_cwd)
                         return True
             except Exception as e:
                 if DEBUG:
@@ -2297,6 +2303,7 @@ def _build_frontend(src_frontend: Path, dest_frontend: Path):
                     if DEBUG:
                         print(f"   ⚠ 'yarn set version' failed: {e}")
             
+            os.chdir(original_cwd)
             return False
 
     # Main logic: Find or install Yarn 4.x