v0.6 — Persistent Registry + TTL

Petri's sandbox registry is currently in-memory — a restart wipes all state. v0.6 fixes that.

What's changing:

• File or DB-backed registry so sandboxes survive restarts

• TTL per sandbox — inactive containers are torn down automatically

• Prevents disk and memory leaks from abandoned sandboxes

Why it matters: The SDK and any long-running agent workflow needs to trust that a sandbox it created is still there. Without persistence, you can't build reliably on top of Petri.