fix: register skills directory via config hook so opencode discovers ponytail skills (#138)

The ponytail plugin was missing a config hook to register its skills/
directory with opencode's skill discovery system. Without this, the 5
ponytail skills (ponytail, ponytail-audit, ponytail-debt, ponytail-help,
ponytail-review) never appear in the skill tool's available list.

The superpowers plugin already follows this exact pattern — this brings
ponytail in line with the upstream convention.