Add ember-best-practices skill

.agents/skills/ember-best-practices/rules/component-strict-mode.md

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+---
+title: Use Strict Mode and Template-Only Components
+impact: HIGH
+impactDescription: Better type safety and simpler components
+tags: strict-mode, template-only, components, gjs
+---
+
+## Use Strict Mode and Template-Only Components
+
+Use strict mode and template-only components for simpler, safer code with better tooling support.
+
+**Incorrect (JavaScript component for simple templates):**
+
+```glimmer-js
+// app/components/user-card.gjs
+import Component from '@glimmer/component';
+
+class UserCard extends Component {
+  <template>
+    <div class="user-card">
+      <h3>{{@user.name}}</h3>
+      <p>{{@user.email}}</p>
+    </div>
+  </template>
+}
+```
+
+**Correct (template-only component):**
+
+```glimmer-js
+// app/components/user-card.gjs
+<template>
+  <div class="user-card">
+    <h3>{{@user.name}}</h3>
+    <p>{{@user.email}}</p>
+  </div>
+</template>
+```
+
+**With TypeScript for better type safety:**
+
+```glimmer-ts
+// app/components/user-card.gts
+import type { TOC } from '@ember/component/template-only';
+
+interface UserCardSignature {
+  Args: {
+    user: {
+      name: string;
+      email: string;
+    };
+  };
+}
+
+const UserCard: TOC<UserCardSignature> = <template>
+  <div class="user-card">
+    <h3>{{@user.name}}</h3>
+    <p>{{@user.email}}</p>
+  </div>
+</template>;
+
+export default UserCard;
+```
+
+**Enable strict mode in your app:**
+
+```javascript
+// ember-cli-build.js
+'use strict';
+
+const EmberApp = require('ember-cli/lib/broccoli/ember-app');
+
+module.exports = function (defaults) {
+  const app = new EmberApp(defaults, {
+    'ember-cli-babel': {
+      enableTypeScriptTransform: true,
+    },
+  });
+
+  return app.toTree();
+};
+```
+
+Template-only components are lighter, more performant, and easier to understand. Strict mode provides better error messages and prevents common mistakes.
+
+Reference: [Ember Strict Mode](https://guides.emberjs.com/release/upgrading/current-edition/templates/)