CodeRabbit pointed out the post-creation guard is structurally
unreachable: immediately after `git checkout -b X origin/$DEFAULT_BRANCH`,
HEAD == origin/$DEFAULT_BRANCH, so both the merge-base form (`MB == DT`)
and the alternative "ahead-of" count form (`AHEAD == 0`) are sentinels
that always pass on a successful fresh checkout. With the explicit base
arg + fail-fast on the checkout, the guard cannot catch anything new.
Removing it (rather than swapping in another no-op that satisfies the
linter but adds no actual coverage) is the honest fix. Comment retained
to explain why no post-creation guard is needed: the explicit base
argument to `git checkout -b` is the single source of correctness for
#2916.
Same simplification mirrored in get-shit-done/workflows/quick.md.
Full suite: 6102/6102.
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