Unify plugin templates and examples in one repository

[akriaueno]

Summary

• move the 11 copyable plugin starters under templates/
• import the tracked snapshot of urth-inc/metatell-official-plugins at 33474cab as ordinary files under examples/
• update the repository documentation, ignore rules, and internal example links for the unified layout
• fix the imported password example's tuple typing so its checks and federated type build succeed

Git history

• keep metatell-plugins as the only Git history
• do not use submodules, subtrees, or an unrelated-history merge
• record source provenance in the import commit and examples/README.md

Validation

• all 11 templates: npm ci, lint, tests when present, and production build
• password-collection-modal: npm ci, lint, Jest with --passWithNoTests, and production build
• external-api-auth: pnpm 9.12.3 frozen-lockfile install, typecheck, 5 backend tests, and production build with non-secret sample configuration
• verified relative README links and syntax for JSON and JavaScript configuration files
• verified there are no nested Git repositories, gitlinks, tracked dependency/build directories, imported environment files, or secret-scanner findings in the current tree

Notes

• the imported examples retain their MIT license under examples/LICENSE
• the templates retain their existing licensing state; selecting a license for them is a separate repository-owner decision
• the existing SonarCloud project key remains unchanged to preserve the current external project mapping
• the SonarCloud check currently fails before analysis because its token cannot query JRE metadata; recent develop runs fail with the same authentication error
• after merge, downstream links can be updated and metatell-official-plugins can be archived separately