fix: eliminate save-state deprecation warning

[ulises-jeremias]

Summary

Upgrades @actions/core and related @actions/* dependencies to their latest compatible versions, eliminating the deprecated save-state command warning in CI. The rebuilt dist/index.js now resolves to a @actions/core whose saveState prefers the GITHUB_STATE environment file over the deprecated save-state command.

Closes #8

Changes

• Upgrade @actions/core ^1.6.0 -> ^1.11.1 (latest v1.x; uses environment files for saveState)
• Upgrade @actions/cache ^4.0.0 -> ^4.1.0
• Upgrade @actions/github ^5.0.0 -> ^5.1.1 (kept on v5 to remain CJS/ncc-compatible)
• Upgrade @actions/tool-cache ^1.7.1 -> ^1.7.2
• Refresh package-lock.json
• Rebuild dist/index.js (npm run build && npm run package) so the checked-in bundle matches source (check-dist passes)

Validation

• npm run lint
• npm run test (14 tests, 3 files)
• npm run build
• npm run package
• npm run format-check (via npm run all)

Built and validated against Node 24 (per .nvmrc).