Two snapshot-store improvements that together address #975 (≈30 GB+ disk consumed by stale snapshot temp files):
1. **Startup cleanup** of `tmp_pack_*` files left by interrupted git pack operations — a side-repo crash or sigkill mid-snapshot can leave hundreds of these orphaned in `.deepseek/snapshots/objects/pack/`.
2. **Prune unreachable objects** during the regular snapshot prune cycle so loose objects from rolled-back snapshots don't accumulate.
Closes#975. Thanks to @axobase001 — the 60-minute staleness threshold is the right balance between cleanup eagerness and not interfering with an actively-running git pack.
axobase001
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