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.

.gitignore

@@ -0,0 +1,64 @@
+# Dependencies
+node_modules/
+.pnp
+.pnp.js
+
+# Testing
+coverage/
+*.lcov
+.nyc_output
+
+# Production build
+dist/
+
+# Models (downloaded at runtime)
+models/
+
+# Runtime data
+brainy-data/
+*.log
+npm-debug.log*
+yarn-debug.log*
+yarn-error.log*
+lerna-debug.log*
+
+# OS files
+.DS_Store
+Thumbs.db
+
+# IDE
+.vscode/
+.idea/
+*.swp
+*.swo
+*~
+
+# Environment
+.env
+.env.local
+.env.*.local
+
+# Debug
+npm-debug.log*
+yarn-debug.log*
+yarn-error.log*
+
+# Cache
+.npm
+.eslintcache
+.cache/
+
+# Optional npm cache directory
+.npm
+
+# Optional eslint cache
+.eslintcache
+
+# Temporary files
+*.tmp
+*.temp
+/tmp/
+
+# Test artifacts
+test-results/
+tests/results/