ProcessKeeper maintains a running ollie-remote process and respawns
it on failure. Multiple agents Dial() the same keeper to get
independent connections. If the process dies, the next Dial()
triggers a respawn (up to 3 attempts with backoff).
All session creation and restore paths now use ProcessKeeper.
Kill/Shutdown close the keeper (which closes the current process).
This fixes 'connection closed' errors persisting indefinitely
after ollie-remote crashes — the next tool call respawns it.
fe2348c0cc