- Add SkillService: scans data/skills/ directory, parses _meta.json and SKILL.md frontmatter
- Add SkillEndpoint: REST API for listing skills and managing agent-skill assignments
- Add agent_skills DB table for agent-to-skill mapping (only relation in DB, skills live on filesystem)
- Extend SystemPromptBuilder: inject assigned skills' SKILL.md content into system prompt
- Extend ChatEndpoint + SessionEndpoint: load agent skills and pass to prompt builder
- Frontend: skill checkboxes in agent form, API functions for skill management
- Add form.css styling for skill list items
