feat: add Ponytail OpenCode skills

[CollieIsCute]

Summary

• add prompt-only Ponytail skills from 5ei74R0/ponytail-skills into ~/.config/opencode/skills
• install each SKILL.md as a chezmoi external file, matching the existing external-file convention and avoiding another archive table targeting the same directory
• include the Ponytail skills-only fork license beside the existing agent-skills license

Source reviewed

• 5ei74R0/ponytail-skills says it is a skills-only fork that keeps Ponytail skill prompts and removes runtime code
• included skills: ponytail, ponytail-review, ponytail-audit, ponytail-debt, ponytail-gain, ponytail-help
• intentionally not included: lifecycle hooks, local Node runtime, MCP server, OpenCode runtime plugin, workflows, npm metadata, benchmark runners

Verification

• fetched upstream README and each referenced skills/*/SKILL.md
• compared branch against master: 1 file changed, only .chezmoiexternal.toml.tmpl
• no package/runtime/hook changes