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/lib/entry.js

@@ -0,0 +1,33 @@
+module.exports = {
+  default: {
+    id: 'oc-pkg',
+    name: 'OC Package',
+    version: '2.0.0',
+    description: 'Test plugin discovered via package.json extensions',
+    register(api) {
+      api.registerTool({
+        name: 'add',
+        description: 'Add two numbers',
+        parameters: {
+          type: 'object',
+          properties: {
+            a: { type: 'number', description: 'First number' },
+            b: { type: 'number', description: 'Second number' },
+          },
+          required: ['a', 'b'],
+        },
+        execute(id, params) {
+          return String(params.a + params.b);
+        },
+      });
+
+      api.registerTool({
+        name: 'info',
+        description: 'Return plugin info',
+        execute(id, params) {
+          return JSON.stringify({ id: this.id, name: this.name, version: this.version });
+        },
+      });
+    },
+  },
+};