Replacement for the active base prompt.
Omit this field to retain the harness profile base or built-in base prompt.
Set it to null to omit the base prompt entirely.
Content placed before the base prompt.
Content placed after the base prompt and before any harness profile suffix.
Structured system prompt configuration for createDeepAgent.
Prompt parts are assembled in the order prefix → base → suffix,
followed by any model-specific harness profile suffix.
