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>

pi-extension/index.js

@@ -120,6 +120,11 @@ export default function ponytailExtension(pi) {
     handler: (_args, ctx) => sendAlias("/skill:ponytail-audit", "", ctx),
   });
 
+  pi.registerCommand("ponytail-gain", {
+    description: "Run /skill:ponytail-gain",
+    handler: (_args, ctx) => sendAlias("/skill:ponytail-gain", "", ctx),
+  });
+
   pi.registerCommand("ponytail-debt", {
     description: "Run /skill:ponytail-debt",
     handler: (_args, ctx) => sendAlias("/skill:ponytail-debt", "", ctx),