feat: add ponytail-audit skill (#20)

* Add ponytail-audit skill

Whole-codebase audit for over-engineering. Scans a repository and produces a
ranked report of things to delete, simplify, or replace with stdlib/native
equivalents.

* Drop counting-by-number heuristics: remove Phase 5 (structural audit) and TODO-age heuristic per review

* feat: slim ponytail-audit and add cross-platform parity

Rewrite the ponytail-audit skill to ponytail-review's lean shape (35 lines,
was 111): same five tags, repo-wide scope, ranked output. Drop the
prescriptive phase walkthrough, sampling rules, report template, and the
license frontmatter that no other skill carries.

Add the adapters the skill was missing so it ships on every supported
platform the way ponytail-review does:
- commands/ponytail-audit.toml
- .opencode/command/ponytail-audit.md
- pi-extension registerCommand (+ test)
- agent-portability and README entries

Co-authored-by: Alexander Brandt <github@a13x.de>
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

---------

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

pi-extension/index.js

@@ -114,6 +114,11 @@ export default function ponytailExtension(pi) {
     handler: (_args, ctx) => sendAlias("/skill:ponytail-review", "", ctx),
   });
 
+  pi.registerCommand("ponytail-audit", {
+    description: "Run /skill:ponytail-audit",
+    handler: (_args, ctx) => sendAlias("/skill:ponytail-audit", "", ctx),
+  });
+
   pi.registerCommand("ponytail-help", {
     description: "Run /skill:ponytail-help",
     handler: (_args, ctx) => sendAlias("/skill:ponytail-help", "", ctx),