feat: sql safety checks

[milton-li]

Summary

This PR adds SQL safety features to the analyzing-data skill.

Features

1. DDL/DML Blocking

• run_sql() and run_sql_pandas() now block dangerous operations:
  • CREATE, DROP, ALTER, TRUNCATE
  • INSERT, UPDATE, DELETE
• Uses sqlglot AST parsing for accurate detection (no false positives from string literals/comments)

2. Automatic LIMIT Injection

• SELECT queries without LIMIT get LIMIT 100 injected at the SQL level
• Improves performance on large tables (stops at 100 rows vs fetching all then truncating)

3. Unsafe Escape Hatch

• New run_sql_unsafe() and run_sql_pandas_unsafe() functions for intentional DDL/DML
• Explicit opt-in required for operations like creating sandbox tables