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/SessionEndpoint.cs

@@ -91,7 +91,7 @@ public static class SessionEndpoint
         ctx.Response.StatusCode = 204;
     }
 
-    private static async Task HandleSystemPrompt(HttpContext ctx, DatabaseService db, MemoryService memoryService)
+    private static async Task HandleSystemPrompt(HttpContext ctx, DatabaseService db, MemoryService memoryService, SkillService skillService)
     {
         var sessionId = ctx.Request.RouteValues["sessionId"]?.ToString();
         if (string.IsNullOrEmpty(sessionId))
@@ -125,7 +125,8 @@ public static class SessionEndpoint
         var markedMessages = await db.GetMarkedMessages(sessionId);
         var agentMemoryDir = memoryService.GetAgentMemoryDir(agent.AgentId);
         var dataDir = memoryService.DataDir;
-        var systemPrompt = SystemPromptBuilder.Build(agent, session, markedMessages, memoryService, agentMemoryDir, dataDir);
+        var agentSkills = await db.GetAgentSkills(agent.AgentId);
+        var systemPrompt = SystemPromptBuilder.Build(agent, session, markedMessages, memoryService, agentMemoryDir, dataDir, agentSkills, skillService);
 
         ctx.Response.ContentType = "text/plain; charset=utf-8";
         await ctx.Response.WriteAsync(systemPrompt);