split Session into Session + AgentState; move all agent-scoped state to AgentState
Session struct now only holds: id, uname, mu, Core, SessionCtx, cancel,
proc, modelsMu/Cache/At.
New AgentState struct holds: log, logVers, ChatOffset, chatCond, plan,
prevPrompt, remote — all per-agent state.
agentHelper now takes an *AgentState alongside *Session. All agent file
operations (plan, log, chat, prevPrompt, offset, remote, cfg) access
AgentState fields instead of Session fields.
Removed startEventLog from NewSession (was writing to session log).
replayMessagesToLog takes *AgentState. Rename no longer appends to log.
ReadPlanStep stubbed out (TODO: look up correct AgentState).
server.go makeStat stubbed out AgentState log size tracking (TODO).
83304e2896