Feature: add a `reuse` tag to ponytail-review/audit for in-repo duplication (ladder rung 2 at review time)

Background

The ladder used at write time (skills/ponytail/SKILL.md) has rung 2, "Already in this codebase? Reuse it, don't re-write it." (added via #253, following #217).

But ponytail-review and ponytail-audit — the skills that check code after it's written, on a diff or on the whole repo — only have five tags: delete, stdlib, native, yagni, shrink. None of them cover "this new helper duplicates one that already exists elsewhere in this same repo." So the write-time rung has no corresponding check-time tag: if a helper gets duplicated anyway, review/audit currently has nothing to catch it with.

Observed case

We had a session that self-reported adding 4 small helpers, although equivalent helpers already existed in the same repo (a Python/JS monorepo). Running the current review tags over that diff would not have flagged it — none of delete/stdlib/native/yagni/shrink matches "equivalent code already exists elsewhere in this repo."

Proposal

Add a sixth tag to both skills/ponytail-review/SKILL.md and skills/ponytail-audit/SKILL.md:

• reuse: an equivalent function/script/config already exists in this repo at <path>. Replacement: point to it.

And add one line to the instructions in both skills: "Search the same repo for an existing equivalent before flagging or accepting a new helper."

Local workaround

Until then, we append one line to the review prompt ourselves: "Search the same repo for an existing equivalent (function/script/config); if found, list it as reuse."

Happy to send a PR if the direction is welcome.