fix: complete OpenClaw plugin compatibility with test coverage

- Add test OpenClaw plugins: oc-simple (openclaw.plugin.json) and oc-pkg (package.json#extensions)
- Fix plugin.go Start() to detect package.json#openclaw.extensions as OC manifest
- Add hasOCExtensions() helper for package.json OC field detection
- Tests: simulator with manifest entry discovery, package.json extension discovery,
  full Plugin.Start() -> loadOCPlugin() -> register -> call pipeline

internal/plugins/openclaw/testdata/oc-pkg/package.json

@@ -0,0 +1,8 @@
+{
+  "name": "oc-pkg",
+  "version": "2.0.0",
+  "description": "OC plugin with package.json entry discovery",
+  "openclaw": {
+    "extensions": ["lib/entry.js"]
+  }
+}