Explain how to recover from an unowned service index

[Brad-Edwards]

An older volume can contain Cortex’s unmarked cortex_6 index. Refusing to adopt it is correct; the current unowned-collision message does not explain recovery.

Done when

• The error safely names the conflicting native index and explains that APTL cannot prove ownership.
• Explain data-preserving investigation/backup first, then aptl lab stop --volumes as an explicit destructive reset option with its data-loss warning.
• Add concise troubleshooting guidance and a regression test for the message.
• Keep refusal to adopt, delete or overwrite an unowned index.