_launch_external_cron_worker wrapped fifty lines of dispatch, payload
write and scope hydration in the routed-fire multiplex context, though
only build_subprocess_env / strip_launch_profile_env read it. Compute
`multiplex_active` once (process flag OR routed fire), serialize it into
the payload, and set the context for exactly the env build inside the
existing secret-scope try/finally. Drop restore_managed_env on this path:
the worker re-runs load_hermes_dotenv -> _apply_managed_env at import
and strip_launch_profile_env already leaves managed keys in place.
_run_job_script now overlays the installed scope and re-applies managed
keys only under multiplex. In a single-profile process the scope is
os.environ, so the overlay could only re-sanitize values the child
already inherits; the "no-op outside multiplex" comment is now literal.
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