Add ember-best-practices skill

.agents/skills/ember-best-practices/rules/template-let-helper.md

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+---
+title: Use {{#let}} to Avoid Recomputation
+impact: MEDIUM
+impactDescription: 30-50% reduction in duplicate work
+tags: templates, helpers, performance, optimization
+---
+
+## Use {{#let}} to Avoid Recomputation
+
+Use `{{#let}}` to compute expensive values once and reuse them in the template instead of calling getters or helpers multiple times.
+
+**Incorrect (recomputes on every reference):**
+
+```glimmer-js
+// app/components/user-card.gjs
+<template>
+  <div class="user-card">
+    {{#if (and this.user.isActive (not this.user.isDeleted))}}
+      <h3>{{this.user.fullName}}</h3>
+      <p>Status: Active</p>
+    {{/if}}
+
+    {{#if (and this.user.isActive (not this.user.isDeleted))}}
+      <button {{on "click" this.editUser}}>Edit</button>
+    {{/if}}
+
+    {{#if (and this.user.isActive (not this.user.isDeleted))}}
+      <button {{on "click" this.deleteUser}}>Delete</button>
+    {{/if}}
+  </div>
+</template>
+```
+
+**Correct (compute once, reuse):**
+
+```glimmer-js
+// app/components/user-card.gjs
+<template>
+  {{#let (and this.user.isActive (not this.user.isDeleted)) as |isEditable|}}
+    <div class="user-card">
+      {{#if isEditable}}
+        <h3>{{this.user.fullName}}</h3>
+        <p>Status: Active</p>
+      {{/if}}
+
+      {{#if isEditable}}
+        <button {{on "click" this.editUser}}>Edit</button>
+      {{/if}}
+
+      {{#if isEditable}}
+        <button {{on "click" this.deleteUser}}>Delete</button>
+      {{/if}}
+    </div>
+  {{/let}}
+</template>
+```
+
+**Multiple values:**
+
+```glimmer-js
+// app/components/checkout.gjs
+<template>
+  {{#let
+    (this.calculateTotal this.items) (this.formatCurrency this.total) (this.hasDiscount this.user)
+    as |total formattedTotal showDiscount|
+  }}
+    <div class="checkout">
+      <p>Total: {{formattedTotal}}</p>
+
+      {{#if showDiscount}}
+        <p>Original: {{total}}</p>
+        <p>Discount Applied!</p>
+      {{/if}}
+    </div>
+  {{/let}}
+</template>
+```
+
+`{{#let}}` computes values once and caches them for the block scope, reducing redundant calculations.