Extend the core session with fields that are genuinely session
concerns currently duplicated in 9p/session:
- Plan (markdown checklist, survives compaction)
- PrevPrompt (last submitted prompt text)
- Peers (bidirectional session links)
The 9p/session layer can delegate to these in a future change.
Defines Session (ID, CWD, state, env, bus, FIFO) as the runtime
environment that hosts an agent. This is the foundation for separating
session concerns from agent reasoning concerns.
Levi Neely