decouple session and agent files in 9P tree
Session-level files (session/{id}/): plan, env, ctl
Agent-level files (session/{id}/agent/{aid}/): cwd, prompt, log, chat,
cfg, state, statewait, usage, cost, ctxsz, models, systemprompt,
tail, prompt.prev, context, toolsrv.loaded, toolsrv.rev, tools
Key changes:
- Split sessionHelper into sessionHelper and agentHelper types
- ctl moved to session level (kill, rn, save, invalidate)
- cwd moved to agent level (read/write)
- Agent cfg no longer supports rename (use session/ctl rn)
- Permissions updated: cwd=0666, ctl=0666, env=0444 at session level
- Removed duplicate envContent method
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