chore(snap-simple-keyring): remove standalone-repo-specific files

Remove files that only make sense in the standalone repo context:
- .github/ (CI workflows, CODEOWNERS — replaced by accounts repo CI)
- .editorconfig, .gitattributes, .nvmrc (standalone dev config)
- .yarn/releases, .yarn/plugins (standalone Yarn 3 runtime)

.yarn/patches/ is kept — patches are applied per-workspace during
yarn install and must be preserved.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: ccharly