Prepare typegap release candidate package checks

[rogerchappel]

Summary

• Add an asserted scripts/package-smoke.mjs that validates npm pack --dry-run --json contents, rejects development leaks, and checks the packaged CLI bin mode and shebang.
• Make npm run build preserve executable CLI permissions and include lint in release:check.
• Add the missing typescript-eslint dev dependency required by the existing ESLint config and remove existing unused imports so lint can run cleanly.
• Update README release verification commands to match the actual gate.

Verification

• npm run release:check
  • npm run check passed
  • npm run lint passed
  • 5 test files passed, 69 tests passed
  • npm run smoke printed CLI help and scanned fixtures
  • npm run package:smoke passed with 27 packed files

Risk Level

• Low. Package/release scripts, lint cleanup, and documentation only; type coverage behavior is unchanged.

Rollback Plan

• Revert this PR to restore the previous release check and package smoke behavior.