chore: replace stale mistral:7b-instruct refs with qwen3.5:9b (#2418)

[mrveiss]

Summary

• Replace all stale mistral:7b-instruct references with qwen3.5:9b across 22 files
• Follows up on Refactor: remove hardcoded mistral:7b-instruct strings, use SSOT constant #2383 which changed the default model
• Covers code (ssot_config.py docstrings/comments) and 21 documentation files
• Backup files left untouched

Closes #2418

Test plan

• grep -r "mistral:7b-instruct" confirms zero remaining refs (excluding backups)
• No runtime behavior changes — docstrings, comments, and docs only
• Pre-commit hooks pass (black, isort, flake8, bandit)

[github-actions]

✅ SSOT Configuration Compliance: Passing

🎉 No hardcoded values detected that have SSOT config equivalents!