Add `pr review <n>` with the three action switches --approve,
--request-changes, and --comment, plus --body/--body-file. Exactly one
action flag is required; zero or multiple raise VALIDATION_ERROR before
any API call, mirroring the merge shorthand-conflict rule. Body
requirements are left to Gitea: a body-less event it rejects surfaces its
422 as VALIDATION_ERROR carrying the server's message. Output is
`review: { number, action }`.
