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.py

@@ -1,13 +1,46 @@
 import os
 import subprocess
+from pathlib import Path
 
 from src.sandbox.local.list_dir import list_dir
 from src.sandbox.sandbox import Sandbox
 
 
 class LocalSandbox(Sandbox):
-    def __init__(self, id: str):
+    def __init__(self, id: str, path_mappings: dict[str, str] | None = None):
+        """
+        Initialize local sandbox with optional path mappings.
+
+        Args:
+            id: Sandbox identifier
+            path_mappings: Dictionary mapping container paths to local paths
+                          Example: {"/mnt/skills": "/absolute/path/to/skills"}
+        """
         super().__init__(id)
+        self.path_mappings = path_mappings or {}
+
+    def _resolve_path(self, path: str) -> str:
+        """
+        Resolve container path to actual local path using mappings.
+
+        Args:
+            path: Path that might be a container path
+
+        Returns:
+            Resolved local path
+        """
+        path_str = str(path)
+
+        # Try each mapping (longest prefix first for more specific matches)
+        for container_path, local_path in sorted(self.path_mappings.items(), key=lambda x: len(x[0]), reverse=True):
+            if path_str.startswith(container_path):
+                # Replace the container path prefix with local path
+                relative = path_str[len(container_path) :].lstrip("/")
+                resolved = str(Path(local_path) / relative) if relative else local_path
+                return resolved
+
+        # No mapping found, return original path
+        return path_str
 
     def execute_command(self, command: str) -> str:
         result = subprocess.run(
@@ -26,16 +59,19 @@ class LocalSandbox(Sandbox):
         return output if output else "(no output)"
 
     def list_dir(self, path: str, max_depth=2) -> list[str]:
-        return list_dir(path, max_depth)
+        resolved_path = self._resolve_path(path)
+        return list_dir(resolved_path, max_depth)
 
     def read_file(self, path: str) -> str:
-        with open(path) as f:
+        resolved_path = self._resolve_path(path)
+        with open(resolved_path) as f:
             return f.read()
 
     def write_file(self, path: str, content: str, append: bool = False) -> None:
-        dir_path = os.path.dirname(path)
+        resolved_path = self._resolve_path(path)
+        dir_path = os.path.dirname(resolved_path)
         if dir_path:
             os.makedirs(dir_path, exist_ok=True)
         mode = "a" if append else "w"
-        with open(path, mode) as f:
+        with open(resolved_path, mode) as f:
             f.write(content)