Document suspected sandbox failure classifier

[yonilerner]

• Adds a design note for an opt-in suspected sandbox-failure classifier to recover from ambiguous Linux ASRT/bubblewrap failures.
• Documents the proposed split between classifier responsibility (sandbox causality + mechanical retry/idempotency risk) and existing escape reviewer responsibility (authorization/security decision).
• Adds a TODO entry linking to the proposal so we can pick it up later.

[yonilerner]

$agent /discuss test