feat(FN-2620): add plugin test-time engine resolution stubs

- Add engine-guard-stub test modules for hermes, openclaw, and paperclip runtime plugins to fail fast on unmocked @fusion/engine imports
- Update each plugin Vitest config to alias @fusion/engine to the local guard stub during test runs
- Keep setup-engine-guard behavior while removing dependency on built engine dist artifacts for plugin-local tests

plugins/fusion-plugin-hermes-runtime/src/__tests__/engine-guard-stub.ts

@@ -0,0 +1,25 @@
+/**
+ * Resolution stub for @fusion/engine in plugin test mode.
+ *
+ * Vitest resolves mocked module IDs before applying vi.mock factories. The
+ * real @fusion/engine workspace package points to dist outputs that are not
+ * built for plugin-local test runs, so we alias to this stub in vitest config.
+ *
+ * Any direct import of @fusion/engine in plugin tests should still fail fast.
+ */
+const guardError = () =>
+  new Error(
+    "Guard: @fusion/engine was imported without an explicit mock. Runtime plugin tests must mock '../pi-module.js' to prevent loading the real engine.",
+  );
+
+export const createFnAgent = () => {
+  throw guardError();
+};
+
+export const promptWithFallback = async () => {
+  throw guardError();
+};
+
+export const describeModel = () => {
+  throw guardError();
+};