feat: add demo.sql golden test file with 5 impactful SQLFluff issues

[sauravsrijan]

Summary

This PR adds a new golden test file (demo.sql) that demonstrates 5 of the most impactful SQLFluff issues for SQL code quality analysis. The file is designed as a production-realistic example showing common SQL anti-patterns.

Key Issues Demonstrated

1. AL03 (aliasing.expression) - Column expressions without explicit aliases
2. AM06 (ambiguous.column_references) - Inconsistent column references in GROUP BY
3. AM09 (ambiguous.order_by_limit) - LIMIT without ORDER BY (non-deterministic results)
4. CV05 (convention.is_null) - NULL comparisons without IS NULL
5. AL01 (aliasing.table) - Implicit table aliasing in FROM clause

Implementation Details

• demo.sql: 48-line SQL file demonstrating all 5 issues in realistic e-commerce analytics queries
• test_demo.py: Comprehensive test suite with 9 tests validating:
  • File existence and size (<100 lines)
  • Detection of each specific issue type
  • Meaningful violation descriptions
  • Multiple issue categories present

Test Results

✅ All 9 tests passing:

• test_demo_file_exists
• test_demo_file_size
• test_al03_aliasing_expression_detected
• test_am06_ambiguous_column_references_detected
• test_am09_limit_without_orderby_detected
• test_cv05_is_null_comparison_detected
• test_al01_implicit_aliasing_detected
• test_at_least_five_issue_types
• test_violations_have_descriptions

Test Plan

• All new tests pass (9/9)
• SQLFluff successfully detects all 5 impactful issues
• File respects 100-line constraint (48 lines)
• Code follows SQL best practices (except for intentional anti-patterns)

🤖 Generated with Claude Code