Allow configuring the default runtime mode

[davisonio]

T3 defaults new threads to Full access. Auto is safer and works with managed Codex policies, but the only control is per-thread and hidden in the composer. Please add a global default runtime mode setting, or remember the last choice.

[JonasDoesThings]

Fully agree that this should be a setting, the same goes for Claude which has basically the same Auto mode as Codex