Sandbox names on the runner provider carry the runner id, because
Managoat.Sandbox hands an adapter nothing but the name: the shape is
runner-<32 hex>-<8 hex>, the runner's UUID without dashes and a random
suffix.
Pure functions. Which runner a new sandbox should be minted on is the host's placement policy and is not here.
Summary
Functions
Mint a sandbox name on the runner: runner-<32 hex>-<8 hex>.
Recover the runner id (as a lowercase dashed UUID) from a sandbox name.