feat: add Angular NgRx best practices documentation

.agents/skills/angular-best-practices-ngrx/AGENTS.md

@@ -0,0 +1,37 @@
+# Angular Ngrx Best Practices
+
+> Use with the core `angular-best-practices` skill.
+
+---
+
+## 1. NgRx State Management
+
+**Impact: HIGH** (Global state)
+
+### 1.1 Keep Reducers Pure
+
+**Impact: HIGH** (Predictable state, testable, time-travel debugging)
+
+Reducers must be pure functions: no side effects, no HTTP calls, no subscriptions. Only compute new state from action and current state. Move side effects to Effects.
+
+**Example:**
+
+```typescript
+on(UsersActions.load, (state) => ({ ...state, loading: true }));
+// HTTP call goes in UsersEffects
+```
+
+### 1.2 Use Feature Selectors
+
+**Impact: MEDIUM** (Memoized selection, better performance)
+
+Use `createFeatureSelector` and `createSelector` for memoized state selection. Selectors only recompute when their inputs change.
+
+**Example:**
+
+```typescript
+const selectCounterState = createFeatureSelector<CounterState>("counter");
+export const selectCount = createSelector(selectCounterState, (s) => s.count);
+```
+
+---