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/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);
     }