feat: add Apple Container support with comprehensive documentation and dev tools

Add native Apple Container support for better performance on macOS while
maintaining full Docker compatibility. Enhance documentation with memory system
details, development guidelines, and sandbox setup instructions. Improve dev
experience with container image pre-pulling and unified cleanup tools.

Key changes:
- Auto-detect and prefer Apple Container on macOS with Docker fallback
- Add APPLE_CONTAINER.md with complete usage and troubleshooting guide
- Document memory system architecture in CLAUDE.md
- Add make setup-sandbox for pre-pulling container images
- Create cleanup-containers.sh for cross-runtime container cleanup
- Update all related documentation (README, SETUP, config examples)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

backend/docs/SETUP.md

@@ -49,6 +49,22 @@ The backend searches for `config.yaml` in this order:
 
 **Recommended**: Place `config.yaml` in project root (`deer-flow/config.yaml`).
 
+## Sandbox Setup (Optional but Recommended)
+
+If you plan to use Docker/Container-based sandbox (configured in `config.yaml` under `sandbox.use: src.community.aio_sandbox:AioSandboxProvider`), it's highly recommended to pre-pull the container image:
+
+```bash
+# From project root
+make setup-sandbox
+```
+
+**Why pre-pull?**
+- The sandbox image (~500MB+) is pulled on first use, causing a long wait
+- Pre-pulling provides clear progress indication
+- Avoids confusion when first using the agent
+
+If you skip this step, the image will be automatically pulled on first agent execution, which may take several minutes depending on your network speed.
+
 ## Troubleshooting
 
 ### Config file not found