loop: introduce StepContext — per-turn context object assembled once

pkg/agent/loop.go

@@ -65,19 +65,31 @@ func run(ctx context.Context, cfg agentConfig, state state) error {
 	// sync.Map is required because execOne may run in concurrent goroutines.
 	var resultCache sync.Map
 
+	// stepCtx carries per-turn context assembled once at the top of each iteration.
+	type StepContext struct {
+		Goal      string
+		TaskState *TaskState
+		Recent    []backend.Message // last N messages (hot window)
+	}
+
 	for {
+		sc := StepContext{
+			Goal:      cfg.preamble,
+			TaskState: state.taskState(),
+			Recent:    state.recentHistory(hotTailSize),
+		}
+
 		var history []backend.Message
-		if ts := state.taskState(); ts != nil {
-			// Structured mode: task state + recent tail only.
-			history = state.recentHistory(hotTailSize)
+		if sc.TaskState != nil {
+			history = sc.Recent
 		} else {
 			history = state.history()
 		}
-		if cfg.preamble != "" {
-			history = append([]backend.Message{{Role: "system", Content: cfg.preamble}}, history...)
+		if sc.Goal != "" {
+			history = append([]backend.Message{{Role: "system", Content: sc.Goal}}, history...)
 		}
-		if ts := state.taskState(); ts != nil {
-			if msg := ts.render(); msg != "" && len(history) > 0 {
+		if sc.TaskState != nil {
+			if msg := sc.TaskState.render(); msg != "" && len(history) > 0 {
 				// Insert after system prompt, before conversation.
 				history = append([]backend.Message{history[0], {Role: "user", Content: msg}}, history[1:]...)
 			}