fix: Add C# language detection to code extraction

Problem: System couldn't extract C# code examples from documentation
because the language detector only recognized C# from CSS classes
but failed to detect C# from code content.

Solution: Added C# heuristic detection patterns:
- 'using System' - System namespace imports
- 'namespace ' - Namespace declarations
- '{ get; set; }' - Property auto-property syntax
- 'public class ' - Public class declarations
- 'private class ' - Private class declarations
- 'internal class ' - Internal class declarations
- 'public static void ' - Static method declarations

Changes:
- cli/doc_scraper.py: Added C# patterns to detect_language() method
- tests/test_scraper_features.py: Added 7 comprehensive C# detection tests

Test Results: 409 passed (+7 new tests), 3 skipped, 0 failed

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

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

cli/doc_scraper.py

@@ -335,6 +335,13 @@ class DocToSkillConverter:
             return 'python'
         if '#include' in code or 'int main' in code:
             return 'cpp'
+        # C# detection
+        if 'using System' in code or 'namespace ' in code:
+            return 'csharp'
+        if '{ get; set; }' in code:
+            return 'csharp'
+        if any(keyword in code for keyword in ['public class ', 'private class ', 'internal class ', 'public static void ']):
+            return 'csharp'
 
         return 'unknown'