fix(pm): check-expected-skips refuses to judge a live head with a roster that is not the base branch's

The roster a live run judges with is this file plus every workflow file in
the checkout it runs from; a head's check-runs come from the workflows of
the merge they were built on. A checkout behind its base therefore read a
job rostered on the base since as an unexpected skip (exit 4). A live run
now compares the roster's sources (this file and every workflow, by git
blob id, working tree against the base branch's live tip via the contents
API: no history, no fetch) before it judges, and answers exit 3 NOT
MEASURED naming each differing path and the re-pin when they differ. The
offline paths (--check-runs-json, --roster) are unchanged.

Claude-Session: https://claude.ai/code/session_01M82feuetLzrCiBz41BJb7L
Co-authored-by: Claude <noreply@anthropic.com>

Author: claude