#30495 fresh-home-launch.sh: fetch ref= from the worker's mirror or GitHub

Follow-up to #30483. Its first fleet run (the fresh-home launch for #30458) exited 3 with 'origin' does not appear to be a git repository, because a fleet step's checkout has no remotes. ref= is now fetched from origin if it exists, then the worker's git mirror, then GitHub, where a PR head can be fetched by sha.

Observed / Not run

• Observed: bash -n passes. The failing run's log shows that the step checkout has no origin, and that the worker keeps a mirror at the path this now tries.
• Not run: the fleet run after this lands. Its result goes on #30458.

Risk: low (an on-demand measure script).

Changelog

none

Agent session: agent session

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