feat(agent): implement agent execution engine and turn handling with background processing

apps/gateway/src/main.rs

@@ -137,9 +137,19 @@ fn run_api_server(port: u16) -> anyhow::Result<()> {
     rt.block_on(async {
         let store = zesdex_domain::core::Store::new();
         store.ensure_dirs()?;
+
+        // Load JWT secret from environment variable with a secure default warning
+        let jwt_secret = std::env::var("JWT_SECRET").unwrap_or_else(|_| {
+            tracing::warn!(
+                "JWT_SECRET environment variable not set; using insecure default. \
+                 Set JWT_SECRET to a secure random value in production."
+            );
+            "dev-secret".to_string()
+        });
+
         let state = zesdex_api::ApiState::new(
             store.base_dir.clone(),
-            "dev-secret",
+            jwt_secret,
             "",
             "deepseek-v4-flash-free",
             Some("https://opencode.ai/zen/v1".to_string()),