system_prompt_suffix: str | None = NoneSUFFIX slot in the prompt assembly order — text appended to
the assembled base system prompt.
Always sits last (after BASE or CUSTOM) so model-tuning guidance
lands closest to the conversation history. None (the default)
means no suffix.
Applied uniformly to every assembled stack that consults this
profile: the main agent, declarative subagents whose model resolves
to this profile, and the auto-added general-purpose subagent. Each
stack receives the suffix on top of its own base prompt
(BASE_AGENT_PROMPT, the subagent's authored prompt, and the GP
base respectively).
See create_deep_agent's system_prompt parameter or
Prompt assembly
for how SUFFIX composes with caller-supplied prompts and
base_system_prompt.