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

Endpoints/ChatEndpoint.cs

@@ -23,7 +23,7 @@ public static class ChatEndpoint
         app.MapPost("/api/chat", HandleChat);
     }
 
-    private static async Task HandleChat(HttpRequest req, HttpResponse res, IHttpClientFactory httpFactory, IConfiguration config, ToolService toolService, DatabaseService db, StreamingTracker streamingTracker, MemoryService memoryService, CancellationToken cancellationToken)
+    private static async Task HandleChat(HttpRequest req, HttpResponse res, IHttpClientFactory httpFactory, IConfiguration config, ToolService toolService, DatabaseService db, StreamingTracker streamingTracker, MemoryService memoryService, SkillService skillService, CancellationToken cancellationToken)
     {
         var body = await JsonSerializer.DeserializeAsync<ChatRequest>(req.Body);
         if (body == null || body.Messages == null || body.Messages.Count == 0)
@@ -80,7 +80,8 @@ public static class ChatEndpoint
         var markedMessages = await db.GetMarkedMessages(sessionId);
         var agentMemoryDir = memoryService.GetAgentMemoryDir(agent.AgentId);
         var dataDir = memoryService.DataDir;
-        var systemPromptText = SystemPromptBuilder.Build(agent, session, markedMessages, memoryService, agentMemoryDir, dataDir);
+        var agentSkills = await db.GetAgentSkills(agent.AgentId);
+        var systemPromptText = SystemPromptBuilder.Build(agent, session, markedMessages, memoryService, agentMemoryDir, dataDir, agentSkills, skillService);
 
         var chainId = Guid.NewGuid().ToString("N");