Two bugbot follow-ups from PR #104:
1. Destroy old runtime handle before creating new one in restore().
When an agent crashes, the tmux session survives. Creating a new
tmux session with the same name fails. Now we destroy the old
handle first (best-effort, non-blocking).
2. Wrap git clone in try-catch in workspace-clone restore(). On clone
failure (network error, disk full), clean up the partial directory
so future restore attempts aren't blocked.
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
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