docs: clarify the git-refs vs git-branch base-path difference

The write-path paragraph described the shared treeWriter as "anchored at base path """, which is both awkward and slightly wrong — basePath is a per-write argument, not a property of the treeWriter. Explain it in words: git-branch writes each checkpoint under its <id[:2]>/<id[2:]>/ shard prefix inside the one v1 tree, while git-refs writes with no prefix so the checkpoint subtree is the root of its own per-checkpoint commit.