Drop an explicit workspace role

Workspace admins (`workspace.members`). The owner's Git provider token is what the git proxy injects. A workspace whose owner left the team has `owner_id` absent and `needs_owner` set; the git proxy refuses it until it has a new owner. Making **someone else** the owner needs their consent: the answer is 202 with a pending `OwnerTransfer` that they accept or decline (`POST /owner-transfers/{transfer}/accept|decline`); until then the old owner stays the owner and the git proxy keeps using the old owner's token. A new offer replaces a pending one to another user (that one is cancelled); offering it again to the same user returns the pending one. The user must be a member of the workspace's team (409 `not-team-member`) and hold at least Editor (409 `owner-ineligible`); accepting checks that again together with their connection to the provider of the origin repository. Taking ownership oneself (`user_id` is the caller) and naming the current owner change it at once and answer 200 with the workspace; taking it cancels a pending transfer. With `promote_to` (decision 63) a Viewer or a team member without a workspace role is promoted to Editor or Admin and offered the ownership in one step.