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/index.js

@@ -119,6 +119,11 @@ export default function ponytailExtension(pi) {
     handler: (_args, ctx) => sendAlias("/skill:ponytail-audit", "", ctx),
   });
 
+  pi.registerCommand("ponytail-debt", {
+    description: "Run /skill:ponytail-debt",
+    handler: (_args, ctx) => sendAlias("/skill:ponytail-debt", "", ctx),
+  });
+
   pi.registerCommand("ponytail-help", {
     description: "Run /skill:ponytail-help",
     handler: (_args, ctx) => sendAlias("/skill:ponytail-help", "", ctx),