Structured system prompt configuration for createDeepAgent.
Prompt parts are assembled in the order prefix → base → suffix,
followed by any model-specific harness profile suffix.
interface SystemPromptConfigReplacement 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.