feat: implement code review improvements for flexible instance configuration

- Add cache-utils.ts with hash memoization, configurable cache, metrics tracking, mutex, and retry logic
- Enhance validation with field-specific error messages in instance-context.ts
- Add JSDoc documentation to all public methods
- Make cache configurable via INSTANCE_CACHE_MAX and INSTANCE_CACHE_TTL_MINUTES env vars
- Add comprehensive test coverage for cache utilities and metrics monitoring
- Fix test expectations for new validation error format

Addresses all feedback from PR #209 code review

🤖 Generated with Claude Code

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

.env.example

@@ -88,6 +88,17 @@ AUTH_TOKEN=your-secure-token-here
 # Maximum number of API request retries (default: 3)
 # N8N_API_MAX_RETRIES=3
 
+# =========================
+# CACHE CONFIGURATION
+# =========================
+# Optional: Configure instance cache settings for flexible instance support
+
+# Maximum number of cached instances (default: 100, min: 1, max: 10000)
+# INSTANCE_CACHE_MAX=100
+
+# Cache TTL in minutes (default: 30, min: 1, max: 1440/24 hours)
+# INSTANCE_CACHE_TTL_MINUTES=30
+
 # =========================
 # OPENAI API CONFIGURATION
 # =========================