Preserve foreign output directories during failed preparation #499

🤖 Generated by the Agentic Engineer

Problem

After the reserved output directory is replaced, a failed copy triggers cleanup against the replacement path and deletes another writer's data.

Proposed direction

Repair the owning observation or output boundary, with a regression that fails on current main before changing the implementation. Preserve the existing conservative behavior and public interface.

Acceptance criteria

• Never recursively delete a mutable output pathname during failure cleanup.
• Preserve a replacement directory and its unrelated files while returning failure.
• Retain partial owned output for explicit recovery and prove source state remains unchanged.

The conservative recovery rule avoids a second pathname race between an ownership check and deletion. Private staging is still cleaned; uncertain output is preserved.

Size: S/M. Part of #101.