Completes the Linear OAuth handshake by exchanging the authorization code for a workspace token. Returns the returnTo path the authorize request carried in the state, when it named one. Returns 409 with code linear_held_by_org when the workspace's organization holds this Linear organization, with code org_holds_other_linear_organization when the organization or a workspace in it holds a different one, and with code linear_organization_in_other_org when another organization holds this one.