Squash merge commits previously landed with only a bare "merge
fusion/fn-XXXX" subject and a single bullet from the branch's commit
log, leaving git log readers without insight into what actually
changed. Now buildDeterministicMergeMessage calls summarizeCommitBody
(title-summarizer lane when configured, default model otherwise) with
the step commits + diffstat, and emits a three-section body: AI summary
+ Commits merged + Files changed. The deterministic sections always
ship so AI failure / timeout still yields a substantive message.
Also extends summarizeCommitBody to take an optional commitLog and
loosens its prompt for more detail when the change warrants it.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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