docs: Apply Japanese version improvements to English TypeScript rules

To maintain consistency between Japanese and English versions, applied the following improvements:

1. Added type guard implementation pattern
   - Clarified concrete usage of unknown type
   - Demonstrated safe validation of external input

2. Clarified class usage criteria
   - Added concrete decision criterion: "Does this data have behavior?"
   - Specified cases requiring state management (ShoppingCart, Session, etc.)

3. Added external API exception to type complexity management
   - External API types are exceptions to 20-field limit
   - Clarified boundary with appropriate internal conversion

Adjusted to English expressions that are clear for Claude Code,
and achieved complete structural alignment between Japanese and English versions (both files are 177 lines).

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: shinpr

docs/rules-en/typescript.md

@@ -21,6 +21,16 @@
 3. **Union Types・Intersection Types**: Combinations of multiple types
 4. **Type Assertions (Last Resort)**: Only when type is certain
 
+**Type Guard Implementation Pattern**
+```typescript
+// Safely validate external input
+function isUser(value: unknown): value is User {
+  return typeof value === 'object' && value !== null && 
+    'id' in value && 'name' in value
+}
+// Usage: if (isUser(data)) { /* data is typed as User */ }
+```
+
 **Modern Type Features**
 - **satisfies Operator**: Type check while preserving type inference
   ```typescript
@@ -54,18 +64,22 @@
 Input Layer (`unknown`) → Type Guard → Business Layer (Type Guaranteed) → Output Layer (Serialization)
 
 **Type Complexity Management**
-- Field Count: Up to 20 (split by responsibility if exceeded)
+- Field Count: Up to 20 (split by responsibility if exceeded, external API types are exceptions)
 - Optional Ratio: Up to 30% (separate required/optional if exceeded)
 - Nesting Depth: Up to 3 levels (flatten if exceeded)
 - Type Assertions: Review design if used 3+ times
+- **External API Types**: Relax constraints and define according to reality (convert appropriately internally)
 
 ## Coding Conventions
 
 **Class Usage Criteria**
+- **Recommended: Implementation with Functions and Interfaces**
+  - Rationale: Improves testability and flexibility of function composition
 - **Classes Allowed**: 
   - Framework requirements (NestJS Controller/Service, TypeORM Entity, etc.)
   - Custom error class definitions
-- **Classes Prohibited**: Use functions and interfaces for everything else
+  - When state and business logic are tightly coupled (e.g., ShoppingCart, Session, StateMachine)
+- **Decision Criterion**: If "Does this data have behavior?" is Yes, consider using a class
   ```typescript
   // ✅ Functions and interfaces
   interface UserService { create(data: UserData): User }