fix: allow prereleases from main candidates

[Obed0101]

Problem

Release run 35916028675 failed in Ensure release runs from main: prerelease candidates were checked only as ancestors of origin/dev, but beta16 was correctly promoted to main via squash and is not an ancestor of dev.

Solution

Allow prerelease candidates to be verified against either origin/dev or origin/main. Stable releases keep the existing main-only guard.

Verification

• git diff --check — passed.
• Candidate ancestry — HEAD is an ancestor of origin/main; the previous failed run confirmed it is not an ancestor of origin/dev.
• PR fix: allow prereleases from main candidates #344 passed all required checks before merging the same one-line fix to dev.
• No deleted paths; this branch changes only .github/workflows/release.yml.

Release follow-up

After this PR merges, rerun the beta16 release workflow from main using source SHA 670d7f60bc2f2601cca7cb0b0558f00a1dd7a4b4.

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