registry/agent/pi: Support persistent sessions across VM restarts

Problem

The Pi adapter always creates in-memory sessions. When the VM is disposed (actor sleep, runner restart, failover) and recreated on the next action, all conversation history is lost. The agent starts fresh with zero context on every VM cycle.

This is a problem for agents that maintain long-running relationships with users and depend on session history being available across restarts.

Expected behavior

Users should be able to configure the Pi adapter to persist sessions to the filesystem so that conversation history survives VM sleep/wake cycles. The mount system already supports writable persistent storage at the agent data directory — sessions written there would naturally survive VM restarts without additional infrastructure.

When configured for persistence, the adapter should resume the most recent session on startup rather than creating an empty one.

Start at the registry/agent/pi adapter and read how sessions are currently created, then inspect the mount system and agent data directory for writable persistent storage. Done means a configurable persistence mode writes sessions there and resumes the most recent session after the VM is recreated.