Add release and promote actions for the shared changesets release flow

[stijnvanhulle]

Summary

• kubb-labs/kubb and kubb-labs/plugins each carried a byte-for-byte identical trio of release scripts (scripts/release.mjs, scripts/createReleases.mjs, scripts/verifyRegistry.mjs) implementing the stage → promote → verify-on-npm → tag → create-GitHub-release flow used by both repos' release.yml.
• Adds two composite actions here so both repos consume one copy instead of two:
  • .github/actions/release wraps the changesets/action call (version + stage-publish via the bundled release.mjs), exposing staged/staged_packages outputs.
  • .github/actions/promote wraps the verify-on-npm / tag / create-GitHub-release sequence (bundling verifyRegistry.mjs, createReleases.mjs, and the format.mjs regex-helper it depends on), exposing an approved output.
• Adds a minimal package.json + vitest + a test.yml workflow so this repo can run the moved test suite (previously there was no package.json or CI test workflow here at all).

Why composite actions instead of a reusable workflow

changesets/action's publish: input is a literal shell command string it runs internally, so it can't be a uses: reference. A composite action step can still nest a uses: changesets/action@... step whose publish: points at ${{ github.action_path }}/release.mjs, i.e. the bundled script's on-runner path. That keeps each consumer's own job structure/permissions/notify steps untouched (they differ between kubb and plugins) while collapsing the actual duplicated logic into one place.

Test plan

• pnpm install && pnpm test passes (18/18 tests, migrated verbatim from kubb's copies).
• Once merged, kubb-labs/kubb#<PR> and kubb-labs/plugins#<PR> (opened alongside this one) pin to this branch's current commit SHA. If this PR is merged with a strategy that changes that SHA (squash/rebase), those two PRs' pinned SHA should be updated to match main before merging them.

Related: kubb-labs/kubb and kubb-labs/plugins PRs consuming these actions (opened alongside this one).

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