Implement Skills feature: filesystem-based skills with agent assignment

- 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

Program.cs

@@ -14,6 +14,7 @@ builder.Services.AddSingleton(new ToolService());
 builder.Services.AddSingleton(new ImageService());
 builder.Services.AddSingleton(new StreamingTracker());
 builder.Services.AddSingleton(new MemoryService(dataDir));
+builder.Services.AddSingleton(new SkillService(dataDir));
 builder.Services.AddSingleton(new DatabaseService(Path.Combine(dataDir, "gott.db")));
 
 var app = builder.Build();
@@ -27,5 +28,6 @@ app.MapAgentEndpoints();
 app.MapWorkspaceEndpoint();
 app.MapInfoEndpoint();
 app.MapBackupEndpoint();
+app.MapSkillEndpoints();
 
 app.Run("http://0.0.0.0:19000");