ci: tell Claude what to watch for when reviewing

Ask for two things on every review: attention to changes in the public
API, with a plain statement of whether compatibility breaks, and review
prose in plain English.

These go in the system prompt rather than in AGENTS.md because the action
restores only CLAUDE.md and .claude from the base branch. A fork's pull
request supplies its own copy of everything else, so guidance that has to
hold while reviewing one can't live there.