Job.launchConfigJson property

Host/operator launch config for the product-template path — the JSON encoding of norsk-ctl’s ProductTemplateLaunchConfig. Daemon-owned and opaque to Manager (stored, forwarded to the worker, never interpreted here). Round-tripped by fromJob/toJob so that updateJob(await jobById(id)) does not erase it. Absent ⇔ unset on the wire; "" is a present-but-empty value.

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