feat(subturn): support stateful iteration for evaluator-optimizer pattern

Add ActualSystemPrompt and InitialMessages fields to SubTurnConfig to enable
stateful worker context passing across multiple evaluation iterations.

Changes:
- Add ActualSystemPrompt field to separate system role from user task description
- Add InitialMessages field to preload ephemeral session history before agent loop starts
- Add Messages field to ToolResult for carrying session history (internal use, not serialized)
- Update runTurn to inject system prompt and preload history from InitialMessages
- Update AgentLoopSpawner to map new fields from tools.SubTurnConfig to agent.SubTurnConfig

This enables the evaluator-optimizer execution strategy in team tool to maintain
worker context across iterations while keeping SubTurn isolation intact.

pkg/agent/loop.go

@@ -64,6 +64,7 @@ type processOptions struct {
 	SenderID                string   // Current sender ID for dynamic context
 	SenderDisplayName       string   // Current sender display name for dynamic context
 	UserMessage             string   // User message content (may include prefix)
+	SystemPromptOverride    string   // Override the default system prompt (Used by SubTurns)
 	Media                   []string // media:// refs from inbound message
 	DefaultResponse         string   // Response when LLM returns empty
 	EnableSummary           bool     // Whether to trigger summarization
@@ -1069,6 +1070,17 @@ func (al *AgentLoop) runAgentLoop(
 	maxMediaSize := cfg.Agents.Defaults.GetMaxMediaSize()
 	messages = resolveMediaRefs(messages, al.mediaStore, maxMediaSize)
 
+	// 1.5 Override the System prompt (e.g., for Evaluator/Optimizer specific personas)
+	if opts.SystemPromptOverride != "" {
+		for i, msg := range messages {
+			if msg.Role == "system" {
+				messages[i].Content = opts.SystemPromptOverride
+				messages[i].SystemParts = []providers.ContentBlock{{Type: "text", Text: opts.SystemPromptOverride}}
+				break
+			}
+		}
+	}
+
 	// 2. Save user message to session
 	if !opts.SkipAddUserMessage && opts.UserMessage != "" {
 		agent.Sessions.AddMessage(opts.SessionKey, "user", opts.UserMessage)