feat: Complete Skill #4 - n8n Validation Expert

Implements comprehensive validation error interpretation and systematic fixing guidance.
Addresses the validation feedback loop problem (15,107 loops from 19,113 errors).

Files created:
- 4 evaluations testing error interpretation, false positives, auto-sanitization, validation loop
- SKILL.md (690 lines) - Validation philosophy, loop pattern, profiles, auto-sanitization
- ERROR_CATALOG.md (865 lines) - 9 error types with examples and fixes
- FALSE_POSITIVES.md (669 lines) - 6 false positive patterns with decision framework
- README.md (329 lines) - Skill metadata with telemetry statistics

Key features:
- Error severity levels (errors/warnings/suggestions)
- Validation loop pattern (2-3 iterations, 23s+58s average)
- 4 validation profiles (minimal/runtime/ai-friendly/strict)
- Auto-sanitization system (operator structure fixes)
- False positive recognition (~40% of warnings acceptable)
- Error distribution analysis (missing_required 45%, invalid_value 28%)
- Recovery strategies (progressive validation, error triage, clean connections)
- Decision framework for warning acceptance

Error catalog:
- missing_required (45%) - Required field not provided
- invalid_value (28%) - Value doesn't match allowed options
- type_mismatch (12%) - Wrong data type
- invalid_expression (8%) - Expression syntax errors
- invalid_reference (5%) - Referenced node doesn't exist
- operator_structure (2%) - Auto-fixed automatically

False positive patterns:
- Missing error handling (acceptable for dev/test)
- No retry logic (acceptable for internal APIs)
- Missing rate limiting (acceptable for low volume)
- Unbounded queries (acceptable for small datasets)
- Missing input validation (acceptable for trusted sources)
- Hardcoded credentials (never acceptable!)

Validation insights:
- 79% of validation errors lead to feedback loops
- Average 2-3 iterations to valid configuration
- 94% success rate within 3 iterations
- ai-friendly profile reduces false positives by 60%
- Auto-sanitization handles operator structure issues automatically

Total: ~2,553 lines across 8 files

Based on analysis of 19,113 validation errors and 15,107 feedback loops.

🤖 Conceived by Romuald Członkowski - https://www.aiadvisors.pl/en

evaluations/validation-expert/eval-003-auto-sanitization.json

@@ -0,0 +1,14 @@
+{
+  "id": "validation-003",
+  "skills": ["n8n-validation-expert"],
+  "query": "I'm getting an error about 'Binary operator cannot have singleValue property'. What does this mean?",
+  "expected_behavior": [
+    "Identifies this as an operator structure issue",
+    "Explains auto-sanitization system",
+    "States that this error will be automatically fixed on next workflow update",
+    "Explains binary vs unary operator distinction",
+    "References the auto-sanitization section",
+    "Suggests that user doesn't need to manually fix this",
+    "May suggest running n8n_update_partial_workflow or validate_workflow to trigger auto-fix"
+  ]
+}