feat: update docs for token efficient usage

frontend/src/core/CLAUDE.md

@@ -0,0 +1,17 @@
+# frontend/src/core/
+
+## Subdirectories
+
+| Directory | What | When to read |
+| --------- | ---- | ------------ |
+| `api/` | API client and request handling | HTTP calls, error handling |
+| `auth/` | Auth0 provider and auth hooks | Authentication setup, user context |
+| `config/` | Runtime configuration | Environment-specific settings |
+| `debug/` | Debug utilities | Development helpers |
+| `error-boundaries/` | React error boundaries | Error handling UI |
+| `hooks/` | Shared React hooks | Reusable hook patterns |
+| `query/` | React Query configuration | Server state management |
+| `store/` | Zustand state management | Client state, global stores |
+| `sync/` | Data synchronization | Offline support, sync logic |
+| `units/` | Unit conversion utilities | Measurement formatting |
+| `utils/` | General utilities | Helper functions |