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/src/sandbox/local/local_sandbox_provider.py

@@ -5,10 +5,42 @@ _singleton: LocalSandbox | None = None
 
 
 class LocalSandboxProvider(SandboxProvider):
+    def __init__(self):
+        """Initialize the local sandbox provider with path mappings."""
+        self._path_mappings = self._setup_path_mappings()
+
+    def _setup_path_mappings(self) -> dict[str, str]:
+        """
+        Setup path mappings for local sandbox.
+
+        Maps container paths to actual local paths, including skills directory.
+
+        Returns:
+            Dictionary of path mappings
+        """
+        mappings = {}
+
+        # Map skills container path to local skills directory
+        try:
+            from src.config import get_app_config
+
+            config = get_app_config()
+            skills_path = config.skills.get_skills_path()
+            container_path = config.skills.container_path
+
+            # Only add mapping if skills directory exists
+            if skills_path.exists():
+                mappings[container_path] = str(skills_path)
+        except Exception as e:
+            # Log but don't fail if config loading fails
+            print(f"Warning: Could not setup skills path mapping: {e}")
+
+        return mappings
+
     def acquire(self, thread_id: str | None = None) -> str:
         global _singleton
         if _singleton is None:
-            _singleton = LocalSandbox("local")
+            _singleton = LocalSandbox("local", path_mappings=self._path_mappings)
         return _singleton.id
 
     def get(self, sandbox_id: str) -> None: