chore(ci): release.yml — add concurrency + pin checkout to v5

[BryanFRD]

Goal

Apply the two-line fix from FerrLabs/.github#56 to this repo's release workflow (typically .github/workflows/release.yml, or the inline release job in ci.yml).

# under the release job (or at workflow top level):
concurrency:
  group: release-${{ github.ref }}
  cancel-in-progress: false

# in the checkout step:
- uses: actions/checkout@v5  # was @v6

Why

• Concurrency: prevents concurrent main merges from racing FerrFlow release runs → loser hits E2006: tag already exists.
• checkout@v5: v6 drops GITHUB_TOKEN on fetch-depth: 0, which silently breaks FerrFlow's git push.

Parent: FerrLabs/.github#57