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/tools/result.go

@@ -1,6 +1,10 @@
 package tools
 
-import "encoding/json"
+import (
+	"encoding/json"
+
+	"github.com/sipeed/picoclaw/pkg/providers"
+)
 
 // ToolResult represents the structured return value from tool execution.
 // It provides clear semantics for different types of results and supports
@@ -34,6 +38,11 @@ type ToolResult struct {
 	// Media contains media store refs produced by this tool.
 	// When non-empty, the agent will publish these as OutboundMediaMessage.
 	Media []string `json:"media,omitempty"`
+
+	// Messages holds the ephemeral session history after execution.
+	// Only populated by SubTurn executions; used by evaluator_optimizer
+	// to carry stateful worker context across evaluation iterations.
+	Messages []providers.Message `json:"-"`
 }
 
 // NewToolResult creates a basic ToolResult with content for the LLM.