Add ember-best-practices skill

.agents/skills/ember-best-practices/rules/component-tracked-toolbox.md

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+---
+title: Use Tracked Toolbox for Complex State
+impact: HIGH
+impactDescription: Cleaner state management
+tags: components, tracked, state-management, performance
+---
+
+## Use Tracked Toolbox for Complex State
+
+For complex state patterns like maps, sets, and arrays that need fine-grained reactivity, use tracked-toolbox utilities instead of marking entire structures as @tracked.
+
+**Incorrect (tracking entire structures):**
+
+```javascript
+import Component from '@glimmer/component';
+import { tracked } from '@glimmer/tracking';
+
+class TodoList extends Component {
+  @tracked items = []; // Entire array replaced on every change
+
+  addItem = (item) => {
+    // Creates new array, invalidates all consumers
+    this.items = [...this.items, item];
+  };
+
+  removeItem = (index) => {
+    // Creates new array again
+    this.items = this.items.filter((_, i) => i !== index);
+  };
+}
+```
+
+**Correct (using tracked-toolbox):**
+
+```javascript
+import Component from '@glimmer/component';
+import { TrackedArray } from 'tracked-built-ins';
+
+class TodoList extends Component {
+  items = new TrackedArray([]);
+
+  // Use arrow functions for methods used in templates (no @action needed)
+  addItem = (item) => {
+    // Efficiently adds to tracked array
+    this.items.push(item);
+  };
+
+  removeItem = (index) => {
+    // Efficiently removes from tracked array
+    this.items.splice(index, 1);
+  };
+}
+```
+
+**Also useful for Maps and Sets:**
+
+```javascript
+import { TrackedMap, TrackedSet } from 'tracked-built-ins';
+
+class Cache extends Component {
+  cache = new TrackedMap(); // Fine-grained reactivity per key
+  selected = new TrackedSet(); // Fine-grained reactivity per item
+}
+```
+
+tracked-built-ins provides fine-grained reactivity and better performance than replacing entire structures.
+
+Reference: [tracked-built-ins](https://github.com/tracked-tools/tracked-built-ins)