fix(gitcafe): diffs still load for pulls that report no head yet

The shared changes-page reader pinned the first page to the pull's headOid,
which GitCafe may report as null, so every diff for such a pull failed. Pin
the head only when the pull names one; version, cursor revision, and
repeated-cursor checks are unchanged. Name the 500-path changes page size.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
1fe6b549aaJulius Marminge committed on 10/25/2026, 7:28:58 AMparentd135b90

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