refactor(redact): remove vestigial shard-scoping from pre-push OPF rewrite

The pre-push rewrite redacts the whole cumulative tree for every unapplied commit (the C1 whole-tree fix), so the shard-scoping path was dead: shardPath was always "", which made shouldDescend/insideShard always return true and left parseShardPathFromCommitMessage uncalled.

Drop the shardPath parameter from collectTreeBlobs and rebuildTreeWithCachedRedaction, delete the unreachable guard branches and the now-unused helpers, and remove the tests that only covered the removed symbols. Behavior is unchanged — both walkers still recurse the whole tree and redact every file except content_hash.txt.