Install any Claude Code skill with `/plugin marketplace add user/repo`

To install any public Claude Code skill, don't clone repos + copy files into ~/.claude/skills/ by hand. Just run `/plugin marketplace add <user>/<repo>` inside Claude Code — it pulls, installs, and activates the skill in one command. Works with any public GitHub repo that ships a valid SKILL.md. Auto-updates on next install. Replaces the old download-copy-restart dance entirely.