After each iteration's _commit_iteration, record the new HEAD SHA and use
it as the diff anchor for the next iteration. Previously capture_diff
always diffed against the initial base commit, causing every iteration to
return the same full cumulative diff — reviewers couldn't see what changed
between iterations, leading to repeated feedback and stuck FAIL loops.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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