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 agent struct receiver was historically 's' (from when it was
called 'session'). Now that session is a separate concept, rename
to 'a' to match the type name.
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