feat: skill v4 — test reflex, ceiling comments, robust-variant rule

Hardening per the A-F benchmark brief: non-trivial logic leaves one
runnable check; ponytail: comments on ceiling-bearing shortcuts name
the ceiling and upgrade path; prefer the edge-case-correct stdlib
option at equal size. Applied to SKILL.md, all cross-agent rule
copies, the hook fallback, and a guard line in ponytail-review.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

skills/ponytail/SKILL.md

@@ -46,7 +46,8 @@ higher one and move on. The first lazy solution that works is the right one.
 - Deletion over addition. Boring over clever — clever is what someone decodes at 3am.
 - Fewest files possible. Shortest working diff wins.
 - Complex request? Ship the lazy version and question it in the same response — "Did X; Y covers it. Need full X? Say so." Never stall on an answer you can default.
-- Mark deliberate simplifications with a `ponytail:` comment (`// ponytail: this exists`) — simple reads as intent, not ignorance.
+- Two stdlib options, same size? Take the one that's correct on edge cases. Lazy means writing less code, not picking the flimsier algorithm.
+- Mark deliberate simplifications with a `ponytail:` comment (`// ponytail: this exists`) — simple reads as intent, not ignorance. Shortcut with a known ceiling (global lock, O(n²) scan, naive heuristic)? The comment names the ceiling and the upgrade path: `# ponytail: global lock — per-account locks if throughput matters`.
 
 ## Output
 
@@ -77,6 +78,12 @@ that prevents data loss, security measures, accessibility basics, anything
 explicitly requested. User insists on the full version → build it, no
 re-arguing.
 
+Non-trivial logic (a branch, a loop, a parser, a money/security path) leaves
+ONE runnable check behind — the smallest thing that fails if the logic
+breaks: an `assert`-based `demo()`/`__main__` self-check or one small
+`test_*.py`. No frameworks, no fixtures, no per-function suites unless
+asked. Trivial one-liners need no test — YAGNI applies to tests too.
+
 ## Boundaries
 
 Ponytail governs what you build, not how you talk (pair with Caveman for