runtime: 'node20' fails sandbox creation on the Vercel backend (HTTP 400)

SandboxOptions.runtime is typed 'node20' | 'node24' (packages/agent-eval/src/lib/sandbox.ts:67), but node20 cannot be created on the Vercel backend. Sandbox.create({ runtime: 'node20', ... }) fails with Status code 400 is not ok before any command runs, while node24 succeeds against the same token, team and project. Confirmed twice.

node20 => FAILED: Status code 400 is not ok
node24 => created ok

Reproduced with @vercel/sandbox directly, so this is the platform rejecting the runtime rather than anything in the wrapper. The Docker backend still builds node20 images fine, so the two backends now disagree about which runtimes exist.

Worth deciding whether to drop node20 from the union, map it forward to node24, or surface a clearer error than a bare 400. Right now an advertised option fails at sandbox creation with nothing pointing at the runtime as the cause.

Found while verifying #177's readFileBuffer across both backends.