Move agent-specific fields into a nested 'reasoning' struct:
- history, runtime, cfg (per-turn params)
- agentName, agentsDir
- baseLayers, promptEnvExtra
- newDispatcher, newBackend
- currentAction, warnedContext, resultCache
The agent struct is now the session host (owns sess + reasoning).
On /agent swap, a new reasoning can be built from the new config
while the session (identity, state, bus, env) remains stable.
The Session type in agent/ is purely a conversation accumulator
(message history, token tracking, compaction state). Rename it to
History to free up 'Session' for the runtime environment concept
in the upcoming agent/session split.
Levi Neely