SetupChainAsync now sequences only non-terminal children (Idle/Queued).
Done/Failed/Cancelled rows are left in place so a re-run on a partially
executed chain keeps history intact and only reshapes the tail. Running
children abort the op since the chain cannot be reshaped mid-flight.
First non-terminal child is explicitly unblocked.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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