fix(review): stop claude -p swallowing the configured skill as a slash command

The composed prompt leads with the configured skill invocation line (e.g. /pr-review-toolkit:review-pr). claude -p expands a leading slash token as a command — observed live in two runs: it resolved to the BUILT-IN /review skill with the entire composed prompt as its arguments, interpolating the configured skill name where a PR number belongs. The configured skill never ran; the built-in maximum-breadth PR-review harness ran instead (its diff-proportional line-by-line finder alone took 11.4 of the run's 15 minutes on a 102KB scope).

Prefix a prose preamble whenever the composed prompt starts with "/", instructing the agent to invoke the skill lines via the Skill tool — expansion is impossible once the first byte isn't a slash. Add Skill to the reviewer allowlist so headless invocation is pre-approved.