fix: use ${CLAUDE_PLUGIN_ROOT} for the variant path and correct the README skill-id claim

Both corrections from the #226 review.

The five language commands listed $HOME/.claude/plugins/marketplaces/... as the
second candidate path, but the plugin runs from
~/.claude/plugins/cache/<marketplace>/<plugin>/<version>/. On the plugin route
that path reads the marketplace clone at master tip rather than the pinned
version, and it does not exist at all under CLAUDE_CONFIG_DIR or the zip cache
route. ${CLAUDE_PLUGIN_ROOT} is substituted by the loader and resolves to the
running plugin directory on every route, matching the existing usage at
commands/plan-doctor.md:11.

README.md:285 still advertised the five variant skill ids as model-invocable.
After the move to skills/i18n/ the plugin scan does not reach them, so only
planning-with-files:planning-with-files is invocable by name on the plugin
route. The translations are reached through their /plan-<lang> commands, or by
installing one as its own skill via npx skills add --skill.