fs: eliminate AgentLog, SessionNode, RootState
State now lives where it belongs:
- Chat log/plan/condvar → agent.Agent (new chatlog.go)
- Models cache → session.Session
- Event handler → agent.Agent.initChatHandler() (self-wiring)
- Message replay → agent.Agent.ReplayMessages()
The fs package is now a pure presentation layer: spec.go (namespace),
support.go (shared utils), newroot.go (entry point), cache.go (ModelCache).
No wrapper types, no parallel registries.
Deleted: AgentLog, SessionNode, RootState, lifecycle.go, agent_log.go,
session_node.go. Removed replay_test.go (needs rewrite against agent pkg).
47b460b2da