Robust SQL extraction: handles explanations, markdown, prefixes

clean_sql now uses 3 strategies in priority order:
1. Extract from ```sql...``` markdown blocks
2. Find first SELECT/WITH/INSERT statement in text
3. Strip leading "sql" keyword fallback

Tested against 5 real model output patterns:
- Clean SQL ✓
- "sql" prefixed ✓
- Markdown fenced ✓
- Explanation before ```sql block ✓
- Explanation with SELECT buried in text ✓

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

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