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/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);