feat(i18n): add Chinese, Japanese, Korean, Russian translations + language selector

Removes LAUNCH.md from public repo (kept locally as gitignored reference;
internal launch playbook with personal posting drafts).

Adds condensed README translations covering value prop, comparison table,
quick start, and usage:
- docs/i18n/README.zh-CN.md  Simplified Chinese
- docs/i18n/README.ja.md     Japanese
- docs/i18n/README.ko.md     Korean
- docs/i18n/README.ru.md     Russian

Adds language selector banner to top of English README. Each translation
has reciprocal selector linking to all five languages.

Translations cover the 4 largest non-English Claude Code user bases. PRs
welcome for additional languages — see CONTRIBUTING.md.

Author: tenxengineer

.gitignore

@@ -26,3 +26,6 @@ build/
 # Claude Code working artifacts (these would be in users' projects, not in this plugin repo)
 .codemap/
 .serena/
+
+# Internal launch playbook — kept local, not published
+LAUNCH.md