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/skills/loader.py

@@ -0,0 +1,77 @@
+from pathlib import Path
+
+from .parser import parse_skill_file
+from .types import Skill
+
+
+def get_skills_root_path() -> Path:
+    """
+    Get the root path of the skills directory.
+
+    Returns:
+        Path to the skills directory (deer-flow/skills)
+    """
+    # backend directory is current file's parent's parent's parent
+    backend_dir = Path(__file__).resolve().parent.parent.parent
+    # skills directory is sibling to backend directory
+    skills_dir = backend_dir.parent / "skills"
+    return skills_dir
+
+
+def load_skills(skills_path: Path | None = None, use_config: bool = True) -> list[Skill]:
+    """
+    Load all skills from the skills directory.
+
+    Scans both public and custom skill directories, parsing SKILL.md files
+    to extract metadata.
+
+    Args:
+        skills_path: Optional custom path to skills directory.
+                     If not provided and use_config is True, uses path from config.
+                     Otherwise defaults to deer-flow/skills
+        use_config: Whether to load skills path from config (default: True)
+
+    Returns:
+        List of Skill objects, sorted by name
+    """
+    if skills_path is None:
+        if use_config:
+            try:
+                from src.config import get_app_config
+
+                config = get_app_config()
+                skills_path = config.skills.get_skills_path()
+            except Exception:
+                # Fallback to default if config fails
+                skills_path = get_skills_root_path()
+        else:
+            skills_path = get_skills_root_path()
+
+    if not skills_path.exists():
+        return []
+
+    skills = []
+
+    # Scan public and custom directories
+    for category in ["public", "custom"]:
+        category_path = skills_path / category
+        if not category_path.exists() or not category_path.is_dir():
+            continue
+
+        # Each subdirectory is a potential skill
+        for skill_dir in category_path.iterdir():
+            if not skill_dir.is_dir():
+                continue
+
+            skill_file = skill_dir / "SKILL.md"
+            if not skill_file.exists():
+                continue
+
+            skill = parse_skill_file(skill_file, category=category)
+            if skill:
+                skills.append(skill)
+
+    # Sort by name for consistent ordering
+    skills.sort(key=lambda s: s.name)
+
+    return skills