Add ember-best-practices skill

.agents/skills/ember-best-practices/rules/route-parallel-model.md

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+---
+title: Parallel Data Loading in Model Hooks
+impact: CRITICAL
+impactDescription: 2-10× improvement
+tags: routes, data-fetching, parallelization, performance
+---
+
+## Parallel Data Loading in Model Hooks
+
+When fetching multiple independent data sources in a route's model hook, use `Promise.all()` or RSVP.hash() to load them in parallel instead of sequentially.
+`export default` in these route examples is intentional because route modules are discovered through resolver lookup. In hybrid `.gjs`/`.hbs` codebases, keep route defaults and add named exports only when you need explicit imports elsewhere.
+
+**Incorrect (sequential loading, 3 round trips):**
+
+```javascript
+// app/routes/dashboard.js
+import Route from '@ember/routing/route';
+import { service } from '@ember/service';
+
+export default class DashboardRoute extends Route {
+  @service store;
+
+  async model() {
+    const user = await this.store.request({ url: '/users/me' });
+    const posts = await this.store.request({ url: '/posts?recent=true' });
+    const notifications = await this.store.request({ url: '/notifications?unread=true' });
+
+    return { user, posts, notifications };
+  }
+}
+```
+
+**Correct (parallel loading, 1 round trip):**
+
+```javascript
+// app/routes/dashboard.js
+import Route from '@ember/routing/route';
+import { service } from '@ember/service';
+import { hash } from 'rsvp';
+
+export default class DashboardRoute extends Route {
+  @service store;
+
+  model() {
+    return hash({
+      user: this.store.request({ url: '/users/me' }),
+      posts: this.store.request({ url: '/posts?recent=true' }),
+      notifications: this.store.request({ url: '/notifications?unread=true' }),
+    });
+  }
+}
+```
+
+Using `hash()` from RSVP allows Ember to resolve all promises concurrently, significantly reducing load time.