Auto-trust cwd on thread start

[aibrahim-oai]

• Persist trusted cwd state during thread/start when the resolved sandbox is elevated.
• Add app-server coverage for trusted root resolution and confirm turn/start does not mutate trust.

[owenlin0]

nit: you can do self.send_internal_error(request_id, format!("failed to persist trusted project state: {err}")).await;