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/parser.py

@@ -0,0 +1,63 @@
+import re
+from pathlib import Path
+
+from .types import Skill
+
+
+def parse_skill_file(skill_file: Path, category: str) -> Skill | None:
+    """
+    Parse a SKILL.md file and extract metadata.
+
+    Args:
+        skill_file: Path to the SKILL.md file
+        category: Category of the skill ('public' or 'custom')
+
+    Returns:
+        Skill object if parsing succeeds, None otherwise
+    """
+    if not skill_file.exists() or skill_file.name != "SKILL.md":
+        return None
+
+    try:
+        content = skill_file.read_text(encoding="utf-8")
+
+        # Extract YAML front matter
+        # Pattern: ---\nkey: value\n---
+        front_matter_match = re.match(r"^---\s*\n(.*?)\n---\s*\n", content, re.DOTALL)
+
+        if not front_matter_match:
+            return None
+
+        front_matter = front_matter_match.group(1)
+
+        # Parse YAML front matter (simple key-value parsing)
+        metadata = {}
+        for line in front_matter.split("\n"):
+            line = line.strip()
+            if not line:
+                continue
+            if ":" in line:
+                key, value = line.split(":", 1)
+                metadata[key.strip()] = value.strip()
+
+        # Extract required fields
+        name = metadata.get("name")
+        description = metadata.get("description")
+
+        if not name or not description:
+            return None
+
+        license_text = metadata.get("license")
+
+        return Skill(
+            name=name,
+            description=description,
+            license=license_text,
+            skill_dir=skill_file.parent,
+            skill_file=skill_file,
+            category=category,
+        )
+
+    except Exception as e:
+        print(f"Error parsing skill file {skill_file}: {e}")
+        return None