Gate asset delete on the release create/refresh condition

[ptr727]

Problem

Closes #217. In build-release-task.yml's github-release job the two paired steps had mismatched conditions:

• Create GitHub release step — if: exists == 'false' || workflow_dispatch
• Delete consumed release asset artifacts step — if: expect_release_assets

On a scheduled run where the release tag already exists (no new commits), the create step is skipped but build-nugetlibrary still uploaded release-asset-<branch>-*, so the delete step removed the freshly built artifacts even though nothing was published/refreshed this run. Low impact (retention-days: 1 means nothing is permanently lost) but the delete's premise ("durable copies now on the release") doesn't hold for that run.

Fix

Gate the delete on the same condition as the create step:

        if: ${{ inputs.expect_release_assets && (steps.release-exists.outputs.exists == 'false' || github.event_name == 'workflow_dispatch') }}

So it only deletes when a release was actually created/refreshed; otherwise the fresh artifacts stay for the run and retention-days: 1 reaps them.

Surfaced via Copilot during the downstream re-sync of #213/#214 (ptr727/LanguageTags#195) and reported upstream rather than diverged locally.

Verification

• actionlint (Docker) clean; CRLF preserved.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR adjusts the build-release-task.yml reusable workflow so the cleanup step that deletes release-asset-<branch>-* workflow artifacts is gated by the same condition as the GitHub release creation/refresh step, avoiding deletion on no-op scheduled runs where the tag already exists.

Changes:

• Gate the "Delete consumed release asset artifacts" step on (release does not exist) OR (manual refresh condition), matching the create-release step logic.
• Expand the surrounding comment to document why the delete is now gated.