chore: remove migration leftovers - #1720

Summary

• Remove obsolete root GraphQL generation and generated types after GitHub operations moved into the adapter workspace.
• Move the GitHub Actions environment contract beside its only test-mock consumer.
• Remove the unreachable core release barrel and unused GitHub Actions schema exports.
• Enable Knip's unused-file check and register the Vitest setup entry explicitly.
• Regenerate the tracked root Action bundle.

Dependency changes

None.

Validation

• npm run ci
• npm run check:clean
• Knip unused-file, type, package-boundary, and generated-bundle checks passed.

Stack

Stacked on #1705. This removes migration residue before check-pr is added in #1706. Runtime behavior does not change.

Review size

• Reviewable changes: +7 -21 across 7 files
• Generated artifacts: +2 -0 across 1 file

GitHub's headline total includes both rows. .gitattributes marks generated artifacts and collapses their diffs by default.