feat: add standalone Pathgrade CLI

[nadavlac]

Summary

• add a scoped standalone Pathgrade command with bundled Vitest and Codex runtimes
• add terminal UX, provenance reporting, diagnostics, and package smoke coverage
• streamline CI and npm staging around one retained, checksum-verified tarball
• remove obsolete release-verification ceremony and Sigstore dependency

Validation

• yarn quality
• package-surface and release-contract tests
• full Vitest suite: 1,361 passed, 18 skipped
• yarn install --immutable
• git diff --check

Notes

The standalone package smoke remained alive after the full Vitest portion completed and was stopped; targeted package and release contracts passed.