After the fs/session refactor, NewEventHandler was defined but never
called. Agents created via session restore or CreateAgent had nil
output handlers, so emit() was a no-op and the chat stream stayed empty.
- Add Agent.SetOutput() to allow setting the event handler post-creation
- Wire up SetOutput(NewEventHandler(al)) in fs/newroot.go for restored sessions
- Wire up SetOutput(NewEventHandler(al)) in fs/handlers.go for new agents
- Also includes expanded UsageStats fields and TaskState persistence