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>

.opencode/command/ponytail-audit.md

@@ -0,0 +1,5 @@
+---
+description: Audit the whole repo for over-engineering, what can be deleted
+---
+
+Audit the entire repository for over-engineering only, not correctness. Scan the whole tree, not a diff. One line per finding, ranked biggest cut first: <tag> <what to cut>. <replacement>. [path]. Tags: delete (dead code/speculative feature), stdlib (reinvented standard library), native (dependency doing what the platform does), yagni (abstraction with one implementation), shrink (same logic, fewer lines). End with the net lines and dependencies removable. If nothing to cut: 'Lean already. Ship.'