DOC: rewrite retry.md — remove duplication, clarify healing section

- Remove repeated "Common Patterns" and "Configuration Reference" sections
- Move self-healing to step-level position (after retryTo, before scenario retries)
- Explain AI healing flow as bullet list showing what happens at each stage
- Link to /heal and /ai instead of duplicating their content

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: DavertMik