The subagent daemon put `model` straight onto claude's argv with no
validation, so an agent could spawn nested sessions on any model the
operator had deliberately kept off its harness. Forward the existing
`hyperhive.availableModels` onto the daemon unit as
HIVE_AVAILABLE_MODELS (same rail `HIVE_TOOL_GROUPS` uses) and check
`start`/`continue` against it before building the config.
Default open: an absent var restricts nothing, so an agent deployed
before this keeps working. An omitted `model` is always allowed — it
lets claude pick its own default rather than naming one.
Refs #4436
46a6317f13