Add contributing guide, issue and PR templates, CI and community files

CONTRIBUTING.md covers setup, the conventions (English comments, every
string in ja / en / zh), where each kind of change lives and how to add a
part, with Japanese and Chinese summaries. Issue forms for bugs, features
and part requests route questions to Discussions and security reports to
private reporting. The PR template carries the typecheck / build /
three-language checklist.

A read-only CI workflow runs typecheck and build on pull requests and on
main; Dependabot groups npm and Actions updates monthly. SECURITY.md and a
Contributor Covenant 2.1 code of conduct complete the community files, and
the README links to them in all three languages.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>