Fix GitHub Pages deploy: Release workflow never deploys

[github-actions]

1. Target

• Closes Fix GitHub Pages deploy: Release workflow never deploys #76

Fix GitHub Pages deployment: the Release workflow has never deployed the site
successfully.

2. Specification / Test Plan

Problem. .github/workflows/release.yml triggered on
pull_request: types:[closed], branches:[release], so the deploy job ran in the
PR-merge ref context (refs/pull/N/merge). The github-pages environment only
allows deployments from main / release, so actions/deploy-pages was
rejected — "Branch refs/pull/63/merge is not allowed to deploy to github-pages
due to environment protection rules." The VitePress build succeeded; only the
deploy step failed, and the workflow had never deployed successfully.

Change (approach C — keep the release-branch gate).

• release.yml: trigger on push: branches:[release] (plus workflow_dispatch)
  instead of pull_request: closed; drop the obsolete
  if: github.event.pull_request.merged == true guards on all three jobs. The
  run then executes in the release branch context.
• github-pages environment: release added to the allowed deployment branches
  (applied in repo settings; not part of this diff).

Aligns with the official VitePress GitHub Pages guide (deploy on push to the
publishing branch).

Verification.

• Once on main and merged to release (a push to release), the Release
  workflow runs from the release context and deploy-pages succeeds (no
  environment-protection rejection).
• handbook.osbrjp.com serves the latest content, incl. /style-guide (currently
  404).
• The build job (pnpm docs:build) already passes.

3. Additional Instructions / Notes for Shipping (optional)

This workflow change only takes effect once it is on the release branch (a
push event uses the workflow from the pushed commit). To publish:

1. Merge this PR into main.
2. Roll back release to its pre-Release Candidate 2026-07-09 20:09:51 +0900 #63 state and re-create the release PR
   (main → release) so it carries this fix and the Style Guide.
3. Merge the release PR → push to release → the Release workflow deploys.

The github-pages environment already allows release.

4. Check before Review Request

• Self Review : I reviewed changes by myself and approved them.
  • Ensure there is no sensitive information, typos, unrelated changes, or debugging code.
• Evidence : I attached evidences to prove the changes.
  • Record and attach a demo video. For minor changes, attaching an image is also acceptable.
  • Evidences should be updated to the latest version when further changes are made.

5. Evidence

The evidence for a deploy-trigger change is the successful Release workflow
run (build + deploy) after the fix reaches release. It cannot be produced
before merge, since the trigger only fires on push to release. The deploy
rejection this fixes is recorded in #76.