The GitHub refresh phase ran synchronously in the entrypoint's notebook
sync, so an offline or slow network could hold container startup for up
to two fetch timeouts (ls-remote + clone, about two minutes at the
defaults) despite the sync being described as non-blocking. The local
template populate and the categorized view still run in the foreground;
the refresh now re-enters itself as a detached child (guarded by a flag
so it forks once), whose phase-1 pass no-ops via the hash state and
whose finalize is idempotent. Verified with an unreachable remote and
an 8 second timeout: the parent returns in under a second with the
notebooks populated while the child owns the waiting.
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