cmdCommitToSubrepo routed each changed file to a sub-repo via
subRepos.find(...) inside the file loop — O(files * repos). Extract a pure
groupFilesBySubrepo() that buckets sub-repos by first path segment and scans
only the matching bucket, dropping it to expected O(files + repos). First-
match-in-array-order semantics (incl. multi-segment sub-repos) are preserved
exactly. Adds the first behavior-lock test for the routing path.
Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>