build: automate changesets releases

[gpbl]

This switches the repo from the manual GitHub Release publish flow to a Changesets-driven release PR flow, while keeping the repo-level safeguards and one-release-per-version GitHub release story.

What Changed

• add a dedicated release workflow that lets Changesets open or update the release PR on pushes to main
• move the repo-specific publish logic into release helper scripts that validate, publish missing package versions, and create one repo GitHub Release per version
• keep the main workflow focused on CI-only checks
• remove the old release-tag validation script that belonged to the manual GitHub Release flow
• document the new release process in .changeset/README.md

Review Notes

• the release PR branch is fixed to changesets-release/main, and only merges from that branch are allowed to publish
• workflow_dispatch is kept for recovery so a rerun can still create the repo GitHub Release after npm publish succeeds
• the publish path still relies on the shared scripts/publish-packages.ts behavior from the prep PR, including skip-if-published handling and public scoped publishes

[changeset-bot]

⚠️ No Changeset found

Latest commit: 89770ab

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

When changesets are added to this PR, you'll see the packages that this PR includes changesets for and the associated semver types

Click here to learn what changesets are, and how to add one.

Click here if you're a maintainer who wants to add a changeset to this PR

[gpbl]

Superseded by the split follow-up PRs:

• build: add changesets release helpers #2948
• build: switch to changesets release workflow #2949

Keeping the smaller stack for review clarity.