fix: narrow the ponytail: marker to real corner-cuts, keep the prefix (#120) (#577)

The ponytail: comment marker was being over-applied to trivial code (#120).
Narrow the rule so only deliberate simplifications that cut a real corner with a
known ceiling get a comment, and keep the `ponytail:` prefix that /ponytail-debt
greps for. Applied across all rule copies, SKILL.md, AGENTS.md, the fallback
instructions, both command files, and the Qoder/OpenCode copies that #491
missed; .openclaw regenerated. helpers.test.js now pins the prefix so it cannot
silently drop again.

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