refactor: move language variants under skills/i18n so the plugin registers one skill

The Claude Code plugin scan reads skills/*/SKILL.md and does not recurse, so
each of the five language variants was registered for every plugin user and
paid its description in every session's system prompt. Measured with
claude plugin details on v3.10.1, that was about 1,010 of the plugin's
roughly 2,254 always-on tokens.

Moving the variants to skills/i18n/planning-with-files-<lang>/ takes them out
of the plugin scan while keeping every skill name, every skill-route install
command and every install destination unchanged: npx skills add resolves
--skill by name over a recursive scan and still installs to
~/.claude/skills/planning-with-files-<lang>/.

The five language slash commands can no longer invoke a plugin-registered
skill by name, so they now read the translated SKILL.md from the skill-route
path or the plugin marketplace path, which keeps the translated body and the
translated templates in play. They also state that the status tokens stay
literal English, since check-complete.sh matches them with grep -F.