Bulk create repos

Register up to 100 remotes in one request. Per-item semantics are exactly POST /repos, and each item names its own workspace_id — so one batch can span workspaces and the receipt is checked per parent.

Partial success: the response is always 207 Multi-Status with one slot per index, even when every item succeeded, because a client that branches on the envelope status before reading the slots will get that branch wrong the first time one item fails. An item's failure carries the same problem body a single-item request would have returned, including 403 for a workspace the caller cannot write.