fix: implement cleanup() to remove the PAT (#30)

[ifauzeee]

Summary

cleanup() in src/main.ts was an empty @todo, even though README.md and action.yml state that the post-job step removes the PAT.

Implemented it (#30):

• Best-effort git config --global --unset-all http.https://github.com/.extraheader (wrapped in try/catch so a missing header or unavailable git is harmless).
• Clear GITHUB_TOKEN via core.exportVariable('GITHUB_TOKEN', '') so subsequent steps cannot reuse the token.

Verification

• npm run build && npm run package succeeds; bundled cleanup() performs the cleanup.

Closes #30

[ulises-jeremias]

Thanks @ifauzeee! Implements the cleanup function that was a // @todo. Verified locally — git config unset + exportVariable works correctly. Merging!

[ulises-jeremias]

Rebased on latest main (after #38 merged) and rebuilt dist/index.js. Pushing the fixed version to a new branch and will link it here.

[ulises-jeremias]

The cleanup() implementation has been merged to main (commit 3e4fb41). Closing this PR as the changes are now in the main branch. Thanks @ifauzeee for the original implementation!