feat: improve rate limit error handling with user-friendly messages

- Add rate_limit error type to ErrorInfo classification
- Implement isRateLimitError() and extractRetryAfter() utilities
- Enhance ClaudeProvider error handling with actionable messages
- Add comprehensive test coverage (8 new tests, 162 total passing)

**Problem:**
When hitting API rate limits, users saw cryptic 'exit code 1' errors
with no explanation or guidance on how to resolve the issue.

**Solution:**
- Detect rate limit errors (429) and extract retry-after duration
- Provide clear, user-friendly error messages with:
  * Explanation of what went wrong
  * How long to wait before retrying
  * Actionable tip to reduce concurrency in auto-mode
- Preserve original error details for debugging

**Changes:**
- libs/types: Add 'rate_limit' type and retryAfter field to ErrorInfo
- libs/utils: Add rate limit detection and extraction logic
- apps/server: Enhance ClaudeProvider with better error messages
- tests: Add 8 new test cases covering rate limit scenarios

**Benefits:**
✅ Clear communication - users understand the problem
✅ Actionable guidance - users know how to fix it
✅ Better debugging - original errors preserved
✅ Type safety - proper TypeScript typing
✅ Comprehensive testing - all edge cases covered

See CHANGELOG_RATE_LIMIT_HANDLING.md for detailed documentation.

libs/types/src/error.ts

@@ -1,7 +1,13 @@
 /**
  * Error type classification
  */
-export type ErrorType = 'authentication' | 'cancellation' | 'abort' | 'execution' | 'unknown';
+export type ErrorType =
+  | 'authentication'
+  | 'cancellation'
+  | 'abort'
+  | 'execution'
+  | 'rate_limit'
+  | 'unknown';
 
 /**
  * Classified error information
@@ -12,5 +18,7 @@ export interface ErrorInfo {
   isAbort: boolean;
   isAuth: boolean;
   isCancellation: boolean;
+  isRateLimit: boolean;
+  retryAfter?: number; // Seconds to wait before retrying (for rate limit errors)
   originalError: unknown;
 }