sandbox: provision-time machine resources for the smolmachines backend (#507)

Stacked on #478 — merge that first.

New smolmachines machines were created at the platform's tiny defaults (1 vCPU / 256MB / 20GB) with no way to size them. Machines are now sized at create time via SMOLMACHINES_CPUS / SMOLMACHINES_MEMORY_MB / SMOLMACHINES_DISK_GB, and the agent-computer profile advertises the configured shape. In-place resize is not wired up because the provider's resize route is not live yet; the test fake records create-time resources and a new test covers the request and the profile.

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