Agents: CRUD, DB table, session binding, replace model picker

- agents table with name, avatar, description, model, persona, instructions, soft-delete
- AgentEndpoint (GET/POST/PUT/DELETE /api/agents)
- sessions + chains now reference agent_id
- ChatEndpoint resolves agent from session (no agent = no chat)
- system prompt built from agent persona + instructions
- frontend: agent editor with avatar upload, agent cards on dashboard
- session: agent dropdown replaces model picker, choice persisted

Endpoints/AgentEndpoint.cs

@@ -0,0 +1,87 @@
+using System.Text.Json;
+using System.Threading.Tasks;
+using Gott.Models;
+using Gott.Services;
+using Microsoft.AspNetCore.Builder;
+using Microsoft.AspNetCore.Http;
+using Microsoft.AspNetCore.Routing;
+
+namespace Gott.Endpoints;
+
+public static class AgentEndpoint
+{
+    public static void MapAgentEndpoints(this IEndpointRouteBuilder app)
+    {
+        app.MapGet("/api/agents", HandleList);
+        app.MapGet("/api/agents/{agentId}", HandleGet);
+        app.MapPost("/api/agents", HandleCreate);
+        app.MapPut("/api/agents/{agentId}", HandleUpdate);
+        app.MapDelete("/api/agents/{agentId}", HandleDelete);
+    }
+
+    private static async Task HandleList(HttpContext ctx, DatabaseService db)
+    {
+        var agents = await db.GetAgents();
+        await ctx.Response.WriteAsJsonAsync(agents);
+    }
+
+    private static async Task HandleGet(HttpContext ctx, DatabaseService db)
+    {
+        var agentId = ctx.Request.RouteValues["agentId"]?.ToString();
+        if (string.IsNullOrEmpty(agentId)) { ctx.Response.StatusCode = 400; return; }
+
+        var agent = await db.GetAgent(agentId);
+        if (agent == null) { ctx.Response.StatusCode = 404; return; }
+
+        await ctx.Response.WriteAsJsonAsync(agent);
+    }
+
+    private static async Task HandleCreate(HttpContext ctx, DatabaseService db)
+    {
+        var body = await JsonSerializer.DeserializeAsync<JsonElement>(ctx.Request.Body);
+
+        var agent = new AgentRecord
+        {
+            Name = body.TryGetProperty("name", out var n) ? n.GetString() ?? "Neuer Agent" : "Neuer Agent",
+            Avatar = body.TryGetProperty("avatar", out var av) ? av.GetString() : null,
+            Description = body.TryGetProperty("description", out var d) ? d.GetString() : null,
+            Model = body.TryGetProperty("model", out var m) ? m.GetString() ?? "sonnet" : "sonnet",
+            Persona = body.TryGetProperty("persona", out var p) ? p.GetString() : null,
+            Instructions = body.TryGetProperty("instructions", out var i) ? i.GetString() : null
+        };
+
+        agent = await db.CreateAgent(agent);
+        await ctx.Response.WriteAsJsonAsync(agent);
+    }
+
+    private static async Task HandleUpdate(HttpContext ctx, DatabaseService db)
+    {
+        var agentId = ctx.Request.RouteValues["agentId"]?.ToString();
+        if (string.IsNullOrEmpty(agentId)) { ctx.Response.StatusCode = 400; return; }
+
+        var agent = await db.GetAgent(agentId);
+        if (agent == null) { ctx.Response.StatusCode = 404; return; }
+
+        var body = await JsonSerializer.DeserializeAsync<JsonElement>(ctx.Request.Body);
+
+        if (body.TryGetProperty("name", out var n)) agent.Name = n.GetString();
+        if (body.TryGetProperty("avatar", out var av)) agent.Avatar = av.GetString();
+        if (body.TryGetProperty("description", out var d)) agent.Description = d.GetString();
+        if (body.TryGetProperty("model", out var m)) agent.Model = m.GetString();
+        if (body.TryGetProperty("persona", out var p)) agent.Persona = p.GetString();
+        if (body.TryGetProperty("instructions", out var i)) agent.Instructions = i.GetString();
+
+        await db.UpdateAgent(agent);
+        agent = await db.GetAgent(agentId);
+        await ctx.Response.WriteAsJsonAsync(agent);
+    }
+
+    private static async Task HandleDelete(HttpContext ctx, DatabaseService db)
+    {
+        var agentId = ctx.Request.RouteValues["agentId"]?.ToString();
+        if (string.IsNullOrEmpty(agentId)) { ctx.Response.StatusCode = 400; return; }
+
+        await db.DeleteAgent(agentId);
+        ctx.Response.StatusCode = 204;
+    }
+}

Endpoints/ChatEndpoint.cs

@@ -39,7 +39,32 @@ public static class ChatEndpoint
             return;
         }
 
-        var modelId = body.Model switch
+        var sessionId = body.SessionId;
+        if (string.IsNullOrEmpty(sessionId))
+        {
+            res.StatusCode = 400;
+            await res.WriteAsJsonAsync(new { error = "sessionId required" });
+            return;
+        }
+
+        // Agent aus Session auflösen
+        var session = await db.GetSession(sessionId);
+        if (session == null || string.IsNullOrEmpty(session.AgentId))
+        {
+            res.StatusCode = 400;
+            await res.WriteAsJsonAsync(new { error = "Kein Agent ausgewählt" });
+            return;
+        }
+
+        var agent = await db.GetAgent(session.AgentId);
+        if (agent == null)
+        {
+            res.StatusCode = 400;
+            await res.WriteAsJsonAsync(new { error = "Agent nicht gefunden" });
+            return;
+        }
+
+        var modelId = agent.Model switch
         {
             "opus" => "claude-opus-4-6",
             "sonnet" => "claude-sonnet-4-6",
@@ -50,20 +75,13 @@ public static class ChatEndpoint
         res.Headers["Cache-Control"] = "no-cache";
         res.Headers["Connection"] = "keep-alive";
 
-        var sessionId = body.SessionId;
-        if (string.IsNullOrEmpty(sessionId))
-        {
-            res.StatusCode = 400;
-            await res.WriteAsJsonAsync(new { error = "sessionId required" });
-            return;
-        }
-
         var chainId = Guid.NewGuid().ToString("N");
 
         await db.CreateChain(new ChainRecord
         {
             ChainId = chainId,
             SessionId = sessionId,
+            AgentId = agent.AgentId,
             Model = modelId,
             SystemPrompt = body.SystemPrompt
         });
@@ -89,13 +107,24 @@ public static class ChatEndpoint
             });
         }
 
-        var systemPrompt = new List<object>
-        {
-            new { type = "text", text = "You are Claude Code, Anthropic's official CLI for Claude." }
-        };
+        var systemPrompt = new List<object>();
+
+        // Agent Persona
+        if (!string.IsNullOrWhiteSpace(agent.Persona))
+            systemPrompt.Add(new { type = "text", text = agent.Persona });
+
+        // Agent Instructions
+        if (!string.IsNullOrWhiteSpace(agent.Instructions))
+            systemPrompt.Add(new { type = "text", text = agent.Instructions });
+
+        // Zusätzlicher System Prompt aus dem UI
         if (!string.IsNullOrWhiteSpace(body.SystemPrompt))
             systemPrompt.Add(new { type = "text", text = body.SystemPrompt });
 
+        // Fallback wenn gar nix gesetzt
+        if (systemPrompt.Count == 0)
+            systemPrompt.Add(new { type = "text", text = "Du bist ein hilfreicher Assistent." });
+
         var anthropic = new AnthropicService(httpFactory, token, toolService, db);
         await anthropic.StreamChat(res, modelId, systemPrompt, messages, chainId);
     }

Endpoints/SessionEndpoint.cs

@@ -61,6 +61,10 @@ public static class SessionEndpoint
         {
             await db.UpdateSessionName(sessionId, nameEl.GetString());
         }
+        if (body.TryGetProperty("agentId", out var agentEl))
+        {
+            await db.UpdateSessionAgent(sessionId, agentEl.GetString());
+        }
 
         var session = await db.GetSession(sessionId);
         await ctx.Response.WriteAsJsonAsync(session);