Override for the default general-purpose subagent system prompt.
None means keep the default system prompt.
HarnessProfile.base_system_promptWhen a profile sets both this field and
HarnessProfile.base_system_prompt, this field wins for the
general-purpose subagent.
The reasoning: general_purpose_subagent.system_prompt is GP-specific
configuration, while base_system_prompt is a global override that
applies to the main agent. The more-specific intent wins on the GP
subagent so a user setting both never sees their GP override
silently dropped. The profile's system_prompt_suffix still
layers on top.