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>