CUSTOM slot in the prompt assembly order — completely replaces
BASE_AGENT_PROMPT as the base prompt when set.
None (the default) means use BASE_AGENT_PROMPT unchanged.
If both base_system_prompt and system_prompt_suffix are set, the
suffix is appended to this custom base. A caller-supplied
system_prompt= is still placed before this base — see
create_deep_agent's system_prompt parameter or
Prompt assembly
for the full assembly order.
Most profiles only set system_prompt_suffix to layer model-tuning
guidance on top of the SDK base.
