sync --ping-reviewers: re-request reviews on locally-changed PRs

[freshtonic]

What to build

A new git queue sync --ping-reviewers flag.

For each PR that is part of the synced queue: if changes were made locally since the previous sync and the PR has reviewers on it, re-request reviews from the same reviewers.

Notes / open questions

• "Changes made locally since the previous sync" — detect via the branch's tip moving (its new pushed head differs from the previously-pushed head), i.e. the branch was actually force-pushed with new content this sync, not merely rebased to an identical tree. Decide whether a pure rebase (same Stable-Commit-Ids, new SHAs, no content change) counts — likely it should NOT re-ping.
• "Has reviewers on it" — requested reviewers and/or past reviewers. Re-request the same set. Use gh API (pulls/{n}/requested_reviewers, and past reviews via pulls/{n}/reviews).
• Only applies to PRs of published queues that sync touches; skip drafts? (open question).
• Flag gating: only ping when --ping-reviewers is passed (opt-in), to avoid surprise notifications.

Acceptance criteria (draft)

• git queue sync --ping-reviewers re-requests reviews only on PRs whose content changed locally this sync.
• Reviewers re-requested match the PR's existing/previous reviewers.
• Without the flag, sync never re-requests reviews.
• A no-op / pure-rebase sync does not ping.