When a step is expanded (e.g., implement -> implement.draft, implement.refine-1,
etc.), any steps that depended on the original step should now depend on the
last step of the expansion.
Call UpdateDependenciesForExpansion after both expand and map operations.
Fixes gt-8tmz.3
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>