fix: use npm exec as primary yarn method (bypasses corepack proxy hang)

npm_install.py

@@ -1993,52 +1993,27 @@ def _build_frontend(src_frontend: Path, dest_frontend: Path):
         if yarn_bin:
             try:
                 result = subprocess.run([yarn_bin, "--version"], capture_output=True, text=True, timeout=3)
-                if DEBUG:
-                    print(f"    debug pick: yarn_bin={yarn_bin}, version={result.stdout.strip()}")
-                major = _get_yarn_major_version(["yarn"])
+                if result.stdout.strip():
+                    major = _get_yarn_major_version(["yarn"])
+                    if DEBUG:
+                        print(f"    debug pick: yarn_bin={yarn_bin}, version={result.stdout.strip()}")
+                    if major and major >= 4:
+                        return ["yarn"]
             except (subprocess.TimeoutExpired, Exception):
                 if DEBUG:
-                    print(f"    debug pick: yarn_bin={yarn_bin} HANGS, removing...")
-                try:
-                    subprocess.run(["rm", "-f", yarn_bin], check=False)
-                    major = None
-                except Exception:
-                    major = None
-            if major and major >= 4:
-                return ["yarn"]
-            elif major:
-                if DEBUG:
-                    print(f"    Found Yarn {major}.x but need 4.x+")
+                    print(f"    debug pick: yarn_bin={yarn_bin} HANGS, skipping")
             
-        # Check yarnpkg
-        if shutil.which("yarnpkg"):
-            major = _get_yarn_major_version(["yarnpkg"])
-            if major and major >= 4:
-                return ["yarnpkg"]
-            elif major:
-                if DEBUG:
-                    print(f"    Found yarnpkg {major}.x but need 4.x+")
-        
-        # Fallback to npm exec
+        # Use npm exec directly (bypasses broken corepack proxy)
         if shutil.which("npm"):
             try:
                 major = _get_yarn_major_version(["npm", "exec", "--yes", "yarn@stable", "--"])
                 if major and major >= 4:
+                    if DEBUG:
+                        print(f"    debug pick: using npm exec for yarn")
                     return ["npm", "exec", "--yes", "yarn@stable", "--"]
             except Exception:
                 pass
 
-        # Fallback to npx
-        if shutil.which("npx"):
-            try:
-                major = _get_yarn_major_version(["npx", "-y", "yarn@stable"])
-                if major and major >= 4:
-                    return ["npx", "-y", "yarn@stable"]
-            except Exception:
-                pass
-        
-        return None
-
     def _cleanup_yarn_artifacts():
         """Remove corrupted yarn artifacts."""
         # Do NOT remove .yarnrc.yml - it configures Yarn 4.x mode