feat(skills): add /ponytail-gain measured-impact scoreboard (#108)

A one-shot scoreboard showing ponytail's measured benchmark impact
(less code, less cost, more speed) as plain ASCII bars, then points to
/ponytail-debt and /ponytail-audit for this repo's real numbers.

Complements the existing skills rather than duplicating them: debt
harvests the ponytail: ledger, audit finds what's cuttable, gain shows
the measured why-it-matters. No per-repo savings number is ever printed
-- the unbuilt version was never written, so there is no real baseline
to subtract from in a live repo. The bars carry the published benchmark
medians (5 tasks, 3 models); per-repo figures come from debt's count.

Ships every adapter the other commands ship: Claude commands/*.toml,
OpenCode .opencode/command/*.md, OpenClaw skill (generated), Pi command
registration. Help card, command enumeration, portability table, and
README updated in the same change.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

scripts/build-openclaw-skills.js

@@ -21,6 +21,7 @@ const DESCRIPTIONS = {
   'ponytail-review': 'Review a diff for over-engineering. Finds what to delete: reinvented stdlib, needless deps, speculative abstractions. One line per finding.',
   'ponytail-audit': 'Audit the whole repo for over-engineering. A ranked list of what to delete, simplify, or replace with stdlib or native features.',
   'ponytail-debt': 'Harvest every ponytail: shortcut comment into one debt ledger, so deferrals get tracked instead of forgotten. One-shot report.',
+  'ponytail-gain': 'Show ponytail measured impact as a scoreboard: less code, less cost, more speed, from the benchmark medians. One-shot display.',
   'ponytail-help': "Quick reference for ponytail's modes, skills, and commands. One-shot display.",
 };