fix(guards): refuse a .npmrc beside a portal manifest, like bunfig.toml

Bun reads .npmrc too, so a registry line beside a borrowed portal's
manifest fetches every dependency from a server the portal's author
picked - the same door bunfig.toml opens, which the installer and guard
check 3 already refuse. Both now reject either file. The #718 integration
test, which pointed Bun at its offline registry through a .npmrc, passes
the registry through NPM_CONFIG_REGISTRY instead; it still proves the
hook stays off, because the hooked package only exists on that server.

Follows GHSA-wq7j-x5jq-pjrw; this case is ours, not the report's. Guard
tests for the workspaces and bunfig.toml refusals, which the previous
commit added without them, come along.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

tests/test_install_portal_clis.py

@@ -83,6 +83,13 @@ class InstallPortalClis(unittest.TestCase):
         (cli / "bunfig.toml").write_text('[install]\nregistry = "https://registry.example"\n', encoding="utf-8")
         self.assertIn("bunfig.toml", unsafe_reason(cli))
 
+    def test_npmrc_is_unsafe(self):
+        # Bun reads .npmrc too: a registry line beside the manifest fetches
+        # every dependency from a server the portal's author picked.
+        cli = self.add_cli("x")
+        (cli / ".npmrc").write_text("registry=https://registry.example/\n", encoding="utf-8")
+        self.assertIn(".npmrc", unsafe_reason(cli))
+
     def test_bun_install_never_runs_lifecycle_scripts(self):
         # #718: --ignore-scripts on every install, so a dependency's hook stays
         # off whatever the Bun release's default trust list says.