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>

docs/agent-portability.md

@@ -33,5 +33,6 @@ instructions, keep its copied rule text aligned with `AGENTS.md`.
 - `skills/ponytail/SKILL.md`: lazy senior dev mode
 - `skills/ponytail-review/SKILL.md`: over-engineering review
 - `skills/ponytail-audit/SKILL.md`: whole-repo over-engineering audit
+- `skills/ponytail-debt/SKILL.md`: harvest `ponytail:` shortcuts into a tracked ledger
 - `skills/ponytail-help/SKILL.md`: quick reference
 - `AGENTS.md`: compact always-on instruction set for agents without skill support