feat: Critical model availability system with multi-source fallback

- Add Model Guardian for critical path verification
- Implement fallback chain: GitHub → CDN → Hugging Face
- Smart detection for Docker, CI, production contexts
- Pre-download option with npm run download-models
- Runtime download with automatic fallback
- Model integrity verification (size, hash)
- Comprehensive deployment documentation

The transformer model (Xenova/all-MiniLM-L6-v2) is critical for operations.
Without it, users cannot access their data. This system ensures it's always
available through multiple redundant sources.

package.json

@@ -142,7 +142,10 @@
     "_workflow:major": "node scripts/release-workflow.js major",
     "_workflow:dry-run": "npm run build && npm test && npm run _release:dry-run",
     "_dry-run": "npm pack --dry-run",
-    "download-models": "node scripts/download-models.cjs"
+    "download-models": "node scripts/download-models.cjs",
+    "prepare-models": "node scripts/prepare-models.js",
+    "models:verify": "node scripts/ensure-models.js",
+    "models:download": "BRAINY_ALLOW_REMOTE_MODELS=true node scripts/download-models.cjs"
   },
   "keywords": [
     "vector-database",