feat(skills): add /ponytail-gain measured-impact scoreboard (#108)

A one-shot scoreboard showing ponytail's measured benchmark impact
(less code, less cost, more speed) as plain ASCII bars, then points to
/ponytail-debt and /ponytail-audit for this repo's real numbers.

Complements the existing skills rather than duplicating them: debt
harvests the ponytail: ledger, audit finds what's cuttable, gain shows
the measured why-it-matters. No per-repo savings number is ever printed
-- the unbuilt version was never written, so there is no real baseline
to subtract from in a live repo. The bars carry the published benchmark
medians (5 tasks, 3 models); per-repo figures come from debt's count.

Ships every adapter the other commands ship: Claude commands/*.toml,
OpenCode .opencode/command/*.md, OpenClaw skill (generated), Pi command
registration. Help card, command enumeration, portability table, and
README updated in the same change.

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

docs/agent-portability.md

@@ -36,5 +36,6 @@ instructions, keep its copied rule text aligned with `AGENTS.md`.
 - `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-gain/SKILL.md`: measured-impact scoreboard from the benchmark
 - `skills/ponytail-help/SKILL.md`: quick reference
 - `AGENTS.md`: compact always-on instruction set for agents without skill support