BASE slot in the prompt assembly order.
When applied to a stack, this replaces its authored base prompt. None
(the default) leaves that base unchanged; the main agent has no authored
base prompt by default.
For the main agent, a caller-supplied system_prompt= (USER) is placed
before this BASE, and system_prompt_suffix (SUFFIX) follows it. See
create_deep_agent's system_prompt parameter or
Prompt assembly
for the full assembly order.
Most profiles only set system_prompt_suffix, so each stack retains its
own base prompt and the main agent's authored base remains empty.
