Create GitHub Releases from package version tags

[patoperpetua]

Goal

release.yml already bumps versions and pushes git tags (@singleton-sd/post-kit-*@x.y.z), but the repository has no GitHub Releases. Enable GitHub Release creation so each package tag has a matching Releases entry with notes.

Context

• Tags exist today; Releases list is empty.
• .release-it.json has github.release: false and npm.publish: false.
• npm publish stays out of scope until packages are ready for public install (after @singleton-sd/post-kit-client / epic Epic: Publish the PostKit client SDK for trusted consumers #4). Do not enable npm in this issue.

Scope

1. After scripts/release-changed.mjs creates and pushes tags in CI, create one GitHub Release per new tag (or one release covering the batch — prefer one Release per package tag, matching how tags are already named).
2. Release body: package name, version bump (from → to), increment type, and a short link to CHANGELOG.md / commits since previous tag when cheap to include.
3. Use built-in GITHUB_TOKEN only (already granted contents: write on release.yml). No new secrets.
4. Update docs/pr-pipelines.md (and .release-it.json / script comments if needed) so the documented release behaviour matches reality.
5. Optional one-shot: document or script how to backfill Releases for existing tags already on main (manual gh release create is fine; do not require rewriting history).

Out of scope

• Publishing packages to npm.
• Changing semver / conventional-commit bump rules.
• Preview environments or SWA.

Constraints

• Public repository: release notes must not include secrets or private business detail.
• Keep commit-then-tags push order; do not reintroduce --follow-tags races.
• Idempotent: re-running must not fail if a Release for that tag already exists (skip or update notes safely).
• Do not touch hub files unrelated to release (pnpm-lock.yaml, app packages) unless the script needs a dependency already present.

Acceptance criteria

• A successful pnpm release:ci run that creates tags also creates matching GitHub Release(s).
• Release titles/bodies identify the package and version clearly.
• npm publish remains disabled.
• docs/pr-pipelines.md documents GitHub Releases as part of the main release flow.
• Existing tags either have a documented backfill path, or a one-time backfill is performed in the same PR / follow-up note on the issue.
• Unit/script tests cover the release-creation helper (mock gh / API) where practical; pnpm test / CI green.

Agent notes

Branch: chore/<issue>-github-releases-from-tags. Read scripts/release-changed.mjs, .github/workflows/release.yml, .release-it.json, and docs/pr-pipelines.md first.