fix(spawn): clear ForUser in sub-turn ToolResult to prevent duplicate messages

[jincheng-xydt]

📝 Description

Clear the ForUser field in the spawn sub-turn ToolResult to prevent duplicate message delivery on async sub-agent completion.

Root cause: When a spawn sub-agent finishes, subturn.go set both ForLLM and ForUser to the same content. This triggered two independent push paths:

1. The async callback in pipeline_execute.go directly pushed to the outbound bus when ForUser was non-empty
2. The main agent's orchestration path also delivered the result via pendingResults

Fix: Clear ForUser (set to "") so the main agent exclusively controls message delivery, eliminating the duplicate.

🗣️ Type of Change

• 🐞 Bug fix (non-breaking change which fixes an issue)

🤖 AI Code Generation

• 🛠️ Mostly AI-generated (AI draft, Human verified/modified)

🔗 Related Issue

Closes #3094

📚 Technical Context (Skip for Docs)

• Reference URL: [Bug] 异步子代理(spawn)任务完成时，ForUser字段被同时用于直接推送和主代理汇总，导致重复消息 #3094
• Reasoning: The issue identified three fix options. Option A (clearing ForUser at the source in subturn.go) was chosen as the most minimal, zero-invasive approach that concentrates the fix at the right abstraction layer.

🧪 Test Environment

• Hardware: PC (x86_64)
• OS: Linux
• Model/Provider: N/A (code logic change)
• Channels: All channels (bug affects all async spawn delivery paths)

☑️ Checklist

• My code/docs follow the style of this project.
• I have performed a self-review of my own changes.
• I have updated the documentation accordingly.

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