feat: add email template library core

[nonggde]

Summary

• Add a folder-local Email Template Library core service with validation, CRUD, search, rendering, and UI-ready state helpers.
• Add synthetic core fixtures, local type contracts, index exports, and a service contract doc.
• Keep the change isolated to tools/v2/individual/email-template-library/ and avoid main app integration or live network calls.

Validation

• node --test tools/v2/individual/email-template-library/tests/template-service.test.mjs
• API export smoke check passed for index.mjs
• git diff --check

Closes #490