chore: tighten rule-copy drift check; align Kiro copy (#9)

Cover the Kiro steering file in the equality check (the stripper is now shared, renamed to stripFrontmatter), and align its body with AGENTS.md: the 'Not lazy about' and 'Non-trivial logic' sentences were split into two paragraphs where the canonical body and the other four copies keep them as one. The improved check caught this on the freshly merged #6.

Also add a small invariant canary: four load-bearing rule phrases must appear verbatim in both SKILL.md (the runtime source of truth) and AGENTS.md, catching a rule that lands in one but not the other even though the two files are intentionally different lengths.

Verified: passes on main, exits 1 when a copy diverges or an invariant goes missing.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

.kiro/steering/ponytail.md

@@ -26,6 +26,4 @@ Rules:
 - Pick the edge-case-correct option when two stdlib approaches are the same size — lazy means less code, not the flimsier algorithm.
 - Mark intentional simplifications with a `ponytail:` comment. If the shortcut has a known ceiling (global lock, O(n²) scan, naive heuristic), the comment names the ceiling and the upgrade path.
 
-Not lazy about: input validation at trust boundaries, error handling that prevents data loss, security, accessibility, anything explicitly requested.
-
-Non-trivial logic leaves ONE runnable check behind — the smallest thing that fails if the logic breaks (an assert-based demo/self-check or one small test file; no frameworks, no fixtures). Trivial one-liners need no test.
+Not lazy about: input validation at trust boundaries, error handling that prevents data loss, security, accessibility, anything explicitly requested. Non-trivial logic leaves ONE runnable check behind — the smallest thing that fails if the logic breaks (an assert-based demo/self-check or one small test file; no frameworks, no fixtures). Trivial one-liners need no test.