[ellen-goc]

Closes #125

Problem

The release workflow's last two runs failed in "Create release":

• 403 on the 🐛 target canonical project board columns in post-success transitions (#123) #124 merge: pull_request events from a fork get a read-only GITHUB_TOKEN regardless of permissions: contents: write, so gh release create v0.2.16 was refused. v0.2.16 was never published.
• 422 on the 🔖 Sync uv.lock to 0.2.15 #118 merge, which had no version bump: "Check if tag already exists" ran git rev-parse v$VERSION, but actions/checkout@v4 fetches no tags. The check always said the tag was missing, so the job tried to re-create v0.2.15.

Change (.github/workflows/release.yml only)

• Trigger on push to main instead of pull_request: closed plus the merged == true guard. A push runs with the repository's own token, whether the merged PR came from this repo or a fork.
• Check for the tag with git ls-remote --exit-code --tags origin refs/tags/<tag>, so a merge without a version bump skips the release instead of failing.

The test and release jobs are otherwise unchanged.

Verification

• actionlint (1.7.12): clean. The YAML parses, and the trigger is push: branches: [main].
• The tag check against this repo's remote: v0.2.15 gives exists=true, v0.2.16 gives exists=false.

Effect on merge

This merge is itself a push to main with pyproject.toml at 0.2.16 and no v0.2.16 tag, so the fixed workflow should publish v0.2.16 automatically. If v0.2.16 has already been created by hand, the run skips the release step.