fix: restore s06 todo planning guidance

[heloowird]

Summary

• restore the s05 todo planning guidance in the s06 main system prompt
• keep the s06 subagent guidance so both behaviors are active

Why

s06 keeps the todo_write tool and nag reminder from s05, but its main SYSTEM prompt only mentioned subagents. That made the inherited todo behavior less discoverable and inconsistent with the s05 progression.

Validation

• python3 -m py_compile s06_subagent/code.py
• python3 -m pytest tests/test_todo_write_string_input.py (not run: pytest is not installed in the current Python environment)

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