ci: add auto-update-pr flywheel workflow

[goastler]

What

Adds auto-update-pr.yml — a post-merge "flywheel" workflow.

After each merge to main (plus a 30-min cron backstop and manual dispatch), it:

• lists open, non-draft PRs targeting main that have auto-merge enabled, oldest first;
• polls each PR's merge state, waiting out GitHub's lazy UNKNOWN computation;
• selects the first PR that is conflict-free and mergeStateStatus == BEHIND (out of date but otherwise green);
• presses the Update branch button (PUT /pulls/{n}/update-branch) on that one PR, then stops.

Updating retriggers the PR's CI; if it passes, auto-merge merges it, which fires another push to main, which updates the next stale PR — a self-sustaining flywheel that unblocks auto-merge PRs stuck on being out of date.

Why one PR per run

Updating every stale PR at once makes them all up-to-date simultaneously: the first merges, the rest go instantly stale again, and a full CI run is burned on each for nothing. Updating one at a time keeps CI spend proportional to merges.

Notes

• Uses PROSOPONATOR_PAT (not the default GITHUB_TOKEN) so the branch update is attributed to a real user and therefore retriggers the PR's CI. The default token would not.
• concurrency: auto-update-pr (no cancel) single-flights runs so simultaneous merges can't double-spend CI.
• Relies on mergeStateStatus == BEHIND, which requires "Require branches to be up to date before merging" to be enabled in branch protection.

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[github-actions]

No updates since 2026-06-23T23:59:11Z (over 7 days), so this PR has been converted to draft. That stops it holding CI runners.

Nothing is lost — gh pr ready 49, or the "Ready for review" button, picks it straight back up. Add the keep-ready label to exempt it permanently.

Converting to draft disables auto-merge, so PRs with auto-merge enabled are excluded from this entirely.

[github-actions]

No updates since 2026-08-14T07:28:06Z (over 7 days), so this PR has been converted to draft. That stops it holding CI runners.

Nothing is lost — gh pr ready 49, or the "Ready for review" button, picks it straight back up. Add the keep-ready label to exempt it permanently.

Converting to draft disables auto-merge, so PRs with auto-merge enabled are excluded from this entirely.

[github-actions]

No updates since 2026-08-22T07:10:56Z (over 7 days), so this PR has been converted to draft. That stops it holding CI runners.

Nothing is lost — gh pr ready 49, or the "Ready for review" button, picks it straight back up. Add the keep-ready label to exempt it permanently.

Converting to draft disables auto-merge, so PRs with auto-merge enabled are excluded from this entirely.