build: switch to changesets release workflow

[gpbl]

This finishes the release migration by wiring the new helper scripts into GitHub Actions and removing the old manual GitHub Release publish path.

What Changed

• add a dedicated release.yml workflow that lets Changesets open or update the release PR and run pnpm release:ci on release merges
• slim .github/workflows/package.yml down to CI-only checks
• remove the old manual-release validation script path
• document the new Changesets-driven release flow in .changeset/README.md

Review Notes

• this PR assumes the helper scripts from the previous PR are already present
• the new workflow keeps a workflow_dispatch recovery path so a rerun can still create the repo GitHub Release after npm publish succeeds
• npm trusted publishing should point at .github/workflows/release.yml once this lands

[changeset-bot]

⚠️ No Changeset found

Latest commit: 599a311

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