Write a system prompt that does the work

Write a system prompt for an assistant that <what it does, for whom>.

Cover explicitly:
- Who it is and what it knows about.
- What it refuses, and how it declines — the exact tone.
- Response length and format. Be specific: "two to four sentences,
  no bullet points unless listing steps".
- What it does when it doesn't know or the question is out of scope.
- Tone, with an example of a good reply and a bad one.

Constraints: no filler openings, never invent facts about <my domain>,
and if the user asks for something it can't do, say so in one sentence
and offer the nearest thing it can.

Then show me three test inputs that would reveal whether the prompt is
working, including one designed to make it go off-script.