check-rule-copies: pin the reuse rung as an invariant (#281)

The reuse rung added in #253 ("Already in this codebase? Reuse it, don't
re-write it.") is load-bearing now, but it was not in the INVARIANTS list,
so a future reword could silently drop it from SKILL.md or AGENTS.md without
tripping the check. Pin it with the substring "in this codebase", present in
both files' rung 2, the same way the safety carve-outs are pinned.

Co-authored-by: Rafaelpta <15382305+Rafaelpta@users.noreply.github.com>