Add ember-best-practices skill

.agents/skills/ember-best-practices/rules/component-minimal-tracking.md

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+---
+title: Avoid Unnecessary Tracking
+impact: HIGH
+impactDescription: 20-40% fewer invalidations
+tags: components, tracked, performance, reactivity
+---
+
+## Avoid Unnecessary Tracking
+
+Only mark properties as `@tracked` if they need to trigger re-renders when changed. Overusing `@tracked` causes unnecessary invalidations and re-renders.
+
+**Incorrect (everything tracked):**
+
+```javascript
+import Component from '@glimmer/component';
+import { tracked } from '@glimmer/tracking';
+import { action } from '@ember/object';
+
+class Form extends Component {
+  @tracked firstName = ''; // Used in template ✓
+  @tracked lastName = ''; // Used in template ✓
+  @tracked _formId = Date.now(); // Internal, never rendered ✗
+  @tracked _validationCache = new Map(); // Internal state ✗
+
+  @action
+  validate() {
+    this._validationCache.set('firstName', this.firstName.length > 0);
+    // Unnecessary re-render triggered
+  }
+}
+```
+
+**Correct (selective tracking):**
+
+```javascript
+import Component from '@glimmer/component';
+import { tracked } from '@glimmer/tracking';
+import { action } from '@ember/object';
+
+class Form extends Component {
+  @tracked firstName = ''; // Rendered in template
+  @tracked lastName = ''; // Rendered in template
+  @tracked isValid = false; // Rendered status
+
+  _formId = Date.now(); // Not tracked - internal only
+  _validationCache = new Map(); // Not tracked - internal state
+
+  @action
+  validate() {
+    this._validationCache.set('firstName', this.firstName.length > 0);
+    this.isValid = this._validationCache.get('firstName');
+    // Only re-renders when isValid changes
+  }
+}
+```
+
+Only track properties that directly affect the template or other tracked getters to minimize unnecessary re-renders.