- _deepen_shallow_repo marks packs in its finally without a git config probe
that could itself raise and turn a finished unshallow into False; markers
are inert in a clone without a promisor remote.
- Installers warn instead of aborting the stage when a marker cannot be
written, matching the Python helper.
- Docs: the manual commands follow HERMES_HOME, and the missing-object
backfill feeds IDs through `fetch --stdin` so a large list cannot exceed
the Windows command-line limit.
- Test and docstring comments describe the contract, not the incident.
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