feat: add ponytail-debt skill (#40)

Closes the last gap from the field review: deferral creep. /ponytail-debt
greps the repo for `ponytail:` comment markers and prints a ledger
(file:line, what was simplified, ceiling, upgrade trigger), flagging any
marker with no trigger as the rot risk. One-shot, reports only.

Full parity like ponytail-audit: skill + commands/.toml + .opencode/.md + pi
registerCommand (+ test) + agent-portability + README.

Verified: tests 32/32 (pi command list updated), rule check green, the scan
finds the repo's real markers, and a live end-to-end run produced a correct
ledger (2 markers, 1 no-trigger, prose/examples excluded).

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-help", "ponytail-review"]);
+  assert.deepEqual([...commands.keys()].sort(), ["ponytail", "ponytail-audit", "ponytail-debt", "ponytail-help", "ponytail-review"]);
 });
 
 test("/ponytail updates session mode and injects instructions", async () => withTempConfig(async () => {
@@ -99,11 +99,13 @@ 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-help").handler("", ctx);
 
   assert.deepEqual(sentUserMessages.map((entry) => entry.text), [
     "/skill:ponytail-review",
     "/skill:ponytail-audit",
+    "/skill:ponytail-debt",
     "/skill:ponytail-help",
   ]);
 });