feat: add skills system for specialized agent workflows (#6)

Implement a skills framework that enables specialized workflows for
specific tasks (e.g., PDF processing, web page generation). Skills are
discovered from the skills/ directory and automatically mounted in
sandboxes with path mapping support.

- Add SkillsConfig for configuring skills path and container mount point
- Implement dynamic skill loading from SKILL.md files with YAML frontmatter
- Add path mapping in LocalSandbox to translate container paths to local paths
- Mount skills directory in AIO Docker sandbox containers
- Update lead agent prompt to dynamically inject available skills
- Add setup documentation and expand config.example.yaml

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

backend/SETUP.md

@@ -0,0 +1,76 @@
+# Setup Guide
+
+Quick setup instructions for DeerFlow.
+
+## Configuration Setup
+
+DeerFlow uses a YAML configuration file that should be placed in the **project root directory**.
+
+### Steps
+
+1. **Navigate to project root**:
+   ```bash
+   cd /path/to/deer-flow
+   ```
+
+2. **Copy example configuration**:
+   ```bash
+   cp config.example.yaml config.yaml
+   ```
+
+3. **Edit configuration**:
+   ```bash
+   # Option A: Set environment variables (recommended)
+   export OPENAI_API_KEY="your-key-here"
+
+   # Option B: Edit config.yaml directly
+   vim config.yaml  # or your preferred editor
+   ```
+
+4. **Verify configuration**:
+   ```bash
+   cd backend
+   python -c "from src.config import get_app_config; print('✓ Config loaded:', get_app_config().models[0].name)"
+   ```
+
+## Important Notes
+
+- **Location**: `config.yaml` should be in `deer-flow/` (project root), not `deer-flow/backend/`
+- **Git**: `config.yaml` is automatically ignored by git (contains secrets)
+- **Priority**: If both `backend/config.yaml` and `../config.yaml` exist, backend version takes precedence
+
+## Configuration File Locations
+
+The backend searches for `config.yaml` in this order:
+
+1. `DEER_FLOW_CONFIG_PATH` environment variable (if set)
+2. `backend/config.yaml` (current directory when running from backend/)
+3. `deer-flow/config.yaml` (parent directory - **recommended location**)
+
+**Recommended**: Place `config.yaml` in project root (`deer-flow/config.yaml`).
+
+## Troubleshooting
+
+### Config file not found
+
+```bash
+# Check where the backend is looking
+cd deer-flow/backend
+python -c "from src.config.app_config import AppConfig; print(AppConfig.resolve_config_path())"
+```
+
+If it can't find the config:
+1. Ensure you've copied `config.example.yaml` to `config.yaml`
+2. Verify you're in the correct directory
+3. Check the file exists: `ls -la ../config.yaml`
+
+### Permission denied
+
+```bash
+chmod 600 ../config.yaml  # Protect sensitive configuration
+```
+
+## See Also
+
+- [Configuration Guide](docs/CONFIGURATION.md) - Detailed configuration options
+- [Architecture Overview](CLAUDE.md) - System architecture