build: add changesets release helpers

[gpbl]

This prepares the release automation move by adding the repo-level helper scripts and tests first, without changing which GitHub Actions workflow publishes packages yet.

What Changed

• add release helper scripts for deciding whether a commit is publishable, publishing missing package versions, and creating the repo GitHub Release
• add a release:ci package script that ties those helpers together
• cover the helper behavior with dedicated script tests

Review Notes

• this PR does not switch the repo to the new release workflow yet; it only introduces the helper code that the follow-up workflow PR will call
• the helper tests mock at the process and network boundaries so the runtime scripts stay focused on the release flow itself

[changeset-bot]

⚠️ No Changeset found

Latest commit: d705399

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

This PR includes no changesets

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