feat: Add n8n-code-javascript skill (Skill #6)

Complete expert guidance for writing JavaScript in n8n Code nodes.

Files added (6 skill files + 5 evaluations):
- SKILL.md (699 lines) - Quick start, mode selection, data access overview
- DATA_ACCESS.md (782 lines) - Complete data access patterns
- COMMON_PATTERNS.md (1,110 lines) - 10 production-tested patterns
- ERROR_PATTERNS.md (763 lines) - Top 5 errors covering 62%+ of failures
- BUILTIN_FUNCTIONS.md (764 lines) - Complete built-in function reference
- README.md (350 lines) - Skill metadata and overview

Total: 4,468 lines across 6 files + 5 evaluation scenarios

Key features:
- Data access patterns: $input.all(), $input.first(), $input.item
- Critical gotcha highlighted: Webhook data under .body
- Return format emphasized: [{json: {...}}]
- Top 5 error patterns with solutions (38%, 8%, 5%, 6% of failures)
- 10 production-tested patterns (multi-source aggregation, regex filtering, etc.)
- Complete built-in function reference ($helpers.httpRequest, DateTime, $jmespath)
- Mode selection guide (All Items vs Each Item)

Evaluations (5):
- eval-001: Webhook body gotcha (most common mistake)
- eval-002: Return format error (missing array wrapper)
- eval-003: HTTP requests with $helpers.httpRequest()
- eval-004: Aggregation pattern with reduce()
- eval-005: Expression syntax confusion (using {{}} in code)

Documentation updates:
- README.md: Updated from 5 to 7 skills
- plugin.json: Added code/javascript/python keywords
- marketplace.json: Updated description to reflect 7 skills

Ready for: Production use

🤖 Generated with Claude Code
Conceived by Romuald Członkowski - https://www.aiadvisors.pl/en

evaluations/code-javascript/eval-004-aggregation-pattern.json

@@ -0,0 +1,25 @@
+{
+  "id": "code-js-004",
+  "skills": ["n8n-code-javascript"],
+  "query": "I need to sum up all the 'amount' values from multiple items in my Code node. How do I access all items and calculate a total?",
+  "expected_behavior": [
+    "Activate n8n-code-javascript skill",
+    "Show $input.all() to get all items",
+    "Demonstrate reduce() function for summing",
+    "Include null handling (item.json.amount || 0)",
+    "Show complete return format with result",
+    "Reference COMMON_PATTERNS.md for aggregation patterns",
+    "Reference DATA_ACCESS.md for $input.all() details",
+    "Emphasize 'Run Once for All Items' mode",
+    "Optionally show additional aggregations (count, average)"
+  ],
+  "expected_content": [
+    "$input.all()",
+    "reduce",
+    "COMMON_PATTERNS.md",
+    "DATA_ACCESS.md",
+    "All Items"
+  ],
+  "priority": "high",
+  "notes": "Aggregation is a very common use case. Tests understanding of $input.all() and array methods."
+}