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/agents/lead_agent/prompt.py

@@ -1,6 +1,8 @@
 from datetime import datetime
 
-SYSTEM_PROMPT = f"""
+from src.skills import load_skills
+
+SYSTEM_PROMPT_TEMPLATE = """
 <role>
 You are DeerFlow 2.0, an open-source super agent.
 </role>
@@ -14,19 +16,16 @@ You are DeerFlow 2.0, an open-source super agent.
 You have access to skills that provide optimized workflows for specific tasks. Each skill contains best practices, frameworks, and references to additional resources.
 
 **Progressive Loading Pattern:**
-1. When a user query matches a skill's use case, immediately call `view` on the skill's main file located at `/mnt/skills/{"{skill_name}"}/SKILL.md`
+1. When a user query matches a skill's use case, immediately call `view` on the skill's main file using the path attribute provided in the skill tag below
 2. Read and understand the skill's workflow and instructions
 3. The skill file contains references to external resources under the same folder
 4. Load referenced resources only when needed during execution
 5. Follow the skill's instructions precisely
 
+**Skills are located at:** {skills_base_path}
+
 <all_available_skills>
-<skill name="generate-web-page">
-Generate a web page or web application
-</skill>
-<skill name="pdf-processing">
-Extract text, fill forms, merge PDFs (pypdf, pdfplumber)
-</skill>
+{skills_list}
 </all_available_skills>
 
 </skill_system>
@@ -64,4 +63,27 @@ All temporary work happens in `/mnt/user-data/workspace`. Final deliverables mus
 
 
 def apply_prompt_template() -> str:
-    return SYSTEM_PROMPT + f"\n<current_date>{datetime.now().strftime('%Y-%m-%d, %A')}</current_date>"
+    # Load all available skills
+    skills = load_skills()
+
+    # Get skills container path from config
+    try:
+        from src.config import get_app_config
+
+        config = get_app_config()
+        container_base_path = config.skills.container_path
+    except Exception:
+        # Fallback to default if config fails
+        container_base_path = "/mnt/skills"
+
+    # Generate skills list XML with paths
+    skills_list = "\n".join(f'<skill name="{skill.name}" path="{skill.get_container_path(container_base_path)}">\n{skill.description}\n</skill>' for skill in skills)
+
+    # If no skills found, provide empty list
+    if not skills_list:
+        skills_list = "<!-- No skills available -->"
+
+    # Format the prompt with dynamic skills
+    prompt = SYSTEM_PROMPT_TEMPLATE.format(skills_list=skills_list, skills_base_path=container_base_path)
+
+    return prompt + f"\n<current_date>{datetime.now().strftime('%Y-%m-%d, %A')}</current_date>"