agent: Remove filesystem event logging, use stdout/stderr only

Omni/Agent.hs currently writes events to _/events/<run-id>.jsonl. This is wrong - agent should make no assumptions about filesystem layout.

Events should go to stdout/stderr based on verbosity level:

• Normal: just final response to stdout
• -v: progress to stderr
• -vv or --json: full events to stdout

The container/orchestrator (agentd) is responsible for capturing and persisting logs, not the agent itself.