fix(memory-core): keep a landed reflection merged when cleanup is incomplete

`finalizeReflectionWorktree` demoted the integration status to `failed`
whenever the cleanup receipt came back incomplete, even after a successful
merge. The cursor was then never advanced and the same content was reflected
again on the next run.

Cleanup is bookkeeping that happens after integration, so the receipt now only
annotates the detail with "Reflection cleanup did not fully complete" and the
outcome the merge produced survives. The leftovers are reclaimed by the
reflection orphan sweep.

Part of #8304