Improve the Planner

[dgageot]

Give it tools to ask questions to the user

[docker-agent]

✅ Review Summary

No issues found in the changed code. The additions look good:

• The user_prompt toolset is properly configured with the correct minimal configuration
• The instruction text appropriately guides the planner agent to use the new tool for asking clarifying questions
• Both the toolset registration and instruction update follow established patterns in the codebase

The user_prompt tool implementation requires no additional configuration parameters beyond type: user_prompt, which is the same pattern used by other simple built-in tools.