AFAICT there is no “format” to agent rule files; it’s just text describing how you’d like the LLM to behave. Typically I’ve seen bullet lists.

Some agents have multiple prompts that are used for different modes; I’ve typically seen this stored as JSON that is agent specific and wouldn’t necessarily apply to different agents.

The only agent specific thing I’ve ever included in a context file is referring to a specific tool. I probably could have abstracted that by describing it as “the tool that does X” or by just telling it to do the function that the tool does.