refactor(cache): remove dead code for undefined marker check (Issue #7)

Removed unreachable code in CacheService.get() method.

Analysis:
- CacheService.set() prevents undefined from being cached (Issue #10 fix)
- Therefore, get() will never encounter '__UNDEFINED__' marker
- Removed 4 lines of dead code checking for this marker

Code Removed:
```typescript
// Check for undefined marker
if (row.value === '__UNDEFINED__') {
  return undefined;
}
```

Replaced With:
- Clarifying comment explaining undefined values are never cached
- Direct JSON.parse() without unnecessary check

Impact:
- Cleaner, more maintainable code
- Eliminates confusion about undefined handling
- Removes unreachable code path
- All 1454 tests still passing

CHANGELOG.md updated with refactoring details.

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

Co-Authored-By: Claude <noreply@anthropic.com>

Author: sjsyrek

CHANGELOG.md

@@ -64,6 +64,16 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   - **Example Attack Prevented**: User creating symlink to `/etc/passwd` and attempting to translate it
   - Location: `src/cli/commands/translate.ts:118-149` (translate method with lstatSync check)
 
+### Refactoring
+- **Dead Code Removal: Undefined Marker** - Cleaned up unreachable code (Issue #7)
+  - Removed undefined marker check from CacheService.get() method
+  - Check was dead code: undefined values are never cached (Issue #10 fix prevents caching undefined)
+  - Removed lines checking `if (row.value === '__UNDEFINED__')`
+  - Added clarifying comment explaining why check is unnecessary
+  - **Impact**: Cleaner, more maintainable code; removes confusion about undefined handling
+  - **Code Quality**: Eliminates unreachable code path that could never be executed
+  - Location: `src/storage/cache.ts:150-152` (get method, removed dead code check)
+
 ### Fixed
 - **Critical: Duplicate text handling in batch translation** - Fixed data loss bug for duplicate inputs
   - When input array contained duplicate texts (e.g., `["Hello", "Hello", "World"]`), only the last occurrence received translation

src/storage/cache.ts

@@ -147,11 +147,8 @@ export class CacheService {
       return null;
     }
 
-    // Check for undefined marker
-    if (row.value === '__UNDEFINED__') {
-      return undefined;
-    }
-
+    // Parse cached value (Issue #7: undefined marker check removed as dead code)
+    // Undefined values are never cached (see set() method Issue #10)
     try {
       return JSON.parse(row.value) as unknown;
     } catch (error) {