Add ember-best-practices skill

.agents/skills/ember-best-practices/rules/component-use-glimmer.md

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+---
+title: Use Glimmer Components Over Classic Components
+impact: HIGH
+impactDescription: 30-50% faster rendering
+tags: components, glimmer, performance, reactivity
+---
+
+## Use Glimmer Components Over Classic Components
+
+Glimmer components are lighter, faster, and have a simpler lifecycle than classic Ember components. They don't have two-way bindings or element lifecycle hooks, making them more predictable and performant.
+
+**Incorrect (classic component):**
+
+```javascript
+// app/components/user-card.js
+import Component from '@ember/component';
+import { computed } from '@ember/object';
+
+export default Component.extend({
+  tagName: 'div',
+  classNames: ['user-card'],
+
+  fullName: computed('user.{firstName,lastName}', function () {
+    return `${this.user.firstName} ${this.user.lastName}`;
+  }),
+
+  didInsertElement() {
+    this._super(...arguments);
+    // Complex lifecycle management
+  },
+});
+```
+
+**Correct (Glimmer component):**
+
+```glimmer-js
+// app/components/user-card.gjs
+import Component from '@glimmer/component';
+
+class UserCard extends Component {
+  get fullName() {
+    return `${this.args.user.firstName} ${this.args.user.lastName}`;
+  }
+
+  <template>
+    <div class="user-card">
+      <h3>{{this.fullName}}</h3>
+      <p>{{@user.email}}</p>
+    </div>
+  </template>
+}
+```
+
+Glimmer components are 30-50% faster, have cleaner APIs, and integrate better with tracked properties.
+
+Reference: [Glimmer Components](https://guides.emberjs.com/release/components/component-state-and-actions/)