feat: add scenario generator with 2K reproducible corpus

Procedural generator (pure Python, no deps) produces incident
scenarios across 10 categories with combinatorial variance:
- 35 MSP environments, 35 time contexts, 20 ransomware variants
- Multiple description builders per category (not one template)
- MITRE ATT&CK references in ~30% of scenarios
- Weighted severity/category distributions matching real MSP data

Regenerated data/scenarios.json with --seed 42 for reproducibility.
The generator IS the provenance — no mystery data blobs.

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

Author: jace-ryan