fix: clear plugin discovery cache after plugin installation (openclaw#39752)

Verified:
- pnpm build
- pnpm check
- pnpm test:macmini

Co-authored-by: GazeKingNuWu <264914544+GazeKingNuWu@users.noreply.github.com>
Co-authored-by: Tak Hoffman <781889+Takhoffman@users.noreply.github.com>

src/cron/isolated-agent/run.test-harness.ts

@@ -64,6 +64,7 @@ vi.mock("../../agents/skills/refresh.js", () => ({
 }));
 
 vi.mock("../../agents/workspace.js", () => ({
+  DEFAULT_IDENTITY_FILENAME: "IDENTITY.md",
   ensureAgentWorkspace: vi.fn().mockResolvedValue({ dir: "/tmp/workspace" }),
 }));