error events now carry phase (startup/preflight/sandbox_init/agent_setup/
agent_loop) and the exception class name (plus its cause), never the message
or trace. Startup and preflight failures that exit(1) before the scan starts
are beaconed with a stable error_type instead of vanishing. scan_ended
distinguishes budget_exceeded, rate_limited, and headless agent_stopped
from user_exit.
