Fix: back-to-back merges cancel the deploy that sends the "now live" comment #96

What happened (BCHC, 2026-10-12): PR #82 (a submission) and #121 (a config change) were merged about a minute apart. pages.yml's concurrency group cancelled the deploy for #82's merge commit, and the deploy for #121 published both. But announce only looks at the PR(s) for its own push, so the submitter of #82 never got the "now live" comment. The issue was labelled status:published (by the submission-status workflow) without the message.

Why it matters: the now-live comment is the retention mechanism for submitters, and a reviewer merging several drafts in a row is the normal case.

Possible fix: have announce cover every merged PR between the last successful deploy's SHA and this one (e.g. compare commits since the previous successful run's head_sha), instead of just this push's commit. Keep the once-per-issue guard so nothing doubles up. Add a test for two merges where the first deploy is cancelled.

Workaround used: posted the now-live message on the issue by hand. Don't re-run the cancelled deploy, since it would publish an older commit.