Drop now-unused ReachesCommit wrapper

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Commit

Stefan Hauboldyesterday

PlanRef now calls CheckAncestry directly, leaving ReachesCommit with no production caller — only a benchmark used it. Point the benchmark at CheckAncestry and remove the wrapper rather than leave dead exported surface.

Co-Authored-By: Claude Opus 4.8 (1M context) noreply@anthropic.com Entire-Checkpoint: cf5b16a42703