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/config/skills_config.py

@@ -0,0 +1,49 @@
+from pathlib import Path
+
+from pydantic import BaseModel, Field
+
+
+class SkillsConfig(BaseModel):
+    """Configuration for skills system"""
+
+    path: str | None = Field(
+        default=None,
+        description="Path to skills directory. If not specified, defaults to ../skills relative to backend directory",
+    )
+    container_path: str = Field(
+        default="/mnt/skills",
+        description="Path where skills are mounted in the sandbox container",
+    )
+
+    def get_skills_path(self) -> Path:
+        """
+        Get the resolved skills directory path.
+
+        Returns:
+            Path to the skills directory
+        """
+        if self.path:
+            # Use configured path (can be absolute or relative)
+            path = Path(self.path)
+            if not path.is_absolute():
+                # If relative, resolve from current working directory
+                path = Path.cwd() / path
+            return path.resolve()
+        else:
+            # Default: ../skills relative to backend directory
+            from src.skills.loader import get_skills_root_path
+
+            return get_skills_root_path()
+
+    def get_skill_container_path(self, skill_name: str, category: str = "public") -> str:
+        """
+        Get the full container path for a specific skill.
+
+        Args:
+            skill_name: Name of the skill (directory name)
+            category: Category of the skill (public or custom)
+
+        Returns:
+            Full path to the skill in the container
+        """
+        return f"{self.container_path}/{category}/{skill_name}"