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/test/extension.test.js

@@ -57,7 +57,7 @@ function withTempConfig(fn) {
 test("extension registers Ponytail commands", () => {
   const { commands } = createPiHarness();
 
-  assert.deepEqual([...commands.keys()].sort(), ["ponytail", "ponytail-audit", "ponytail-debt", "ponytail-help", "ponytail-review"]);
+  assert.deepEqual([...commands.keys()].sort(), ["ponytail", "ponytail-audit", "ponytail-debt", "ponytail-gain", "ponytail-help", "ponytail-review"]);
 });
 
 test("/ponytail updates session mode and injects instructions", async () => withTempConfig(async () => {
@@ -100,12 +100,14 @@ test("skill alias commands delegate to Pi skill commands", async () => {
   await commands.get("ponytail-review").handler("", ctx);
   await commands.get("ponytail-audit").handler("", ctx);
   await commands.get("ponytail-debt").handler("", ctx);
+  await commands.get("ponytail-gain").handler("", ctx);
   await commands.get("ponytail-help").handler("", ctx);
 
   assert.deepEqual(sentUserMessages.map((entry) => entry.text), [
     "/skill:ponytail-review",
     "/skill:ponytail-audit",
     "/skill:ponytail-debt",
+    "/skill:ponytail-gain",
     "/skill:ponytail-help",
   ]);
 });