When the engine aborted in-flight work mid-execution and bounced the task
back to `todo`, the single-session teardown cleared the task `branch` and
re-queued without `preserveResumeState` — resetting every step to `pending`
(store.moveTaskInternal resetAllStepsToPending) and dropping the pointer to
the commits already on the task branch. The next dispatch then re-planned
from Step 0 and the committed work was stranded, observed as FN-6722 losing
all its progress and getting stuck in todo.
The teardown now keeps the branch and moves with `preserveResumeState`
whenever the task has resumable step progress, matching the sibling
step-session (executor ~8065) and pause-park (executor ~1826) paths, so
execute() resumes onto the existing branch from the first incomplete step.
The worktree is still removed to free its concurrency slot (FN-6782) — only
the durable pointers (branch + step state) are kept.
Adds a regression test driving the exact single-session catch teardown.
Fusion-Task-Id: FN-6722
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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