Add a disposal lifecycle for the Flue agentOS Core sandbox adapter

[NathanFlurry]

Problem

Flue's SandboxFactory creates or reconnects a sandbox for a context, but it does not expose a lifecycle hook when that context is permanently disposed.

The standalone agentOSCoreSandbox() adapter therefore cannot distinguish an idle context that Flue may reuse from a finished context whose Core VM can be safely disposed. The current safe behavior leaves Core VM lifetime under caller ownership, which can retain resources indefinitely when callers do not manage that lifecycle explicitly.

This affects only the standalone agentOS Core adapter. The regular actor-backed agentOSSandbox() integration is unaffected because a stable Flue context maps to a Rivet Actor and Rivet owns the VM lifecycle and durable filesystem.

Desired outcome

• Coordinate or add a Flue sandbox/context disposal lifecycle hook.
• Dispose the corresponding standalone Core VM when Flue permanently releases a context.
• Add focused lifecycle coverage.
• Remove the caller-owned lifecycle caveat from the Flue guide once the hook is available.