docs: fold in remaining live fixes from overnight-sweep PR cluster
Salvaged from the same PR family (#50691, #59335, #67832 by @virtuadex):
- sessions.md: gateway routing index is the gateway_routing table in
state.db; sessions.json is a legacy mirror behind
gateway.write_sessions_json (from #59335)
- mcp.md: MCP server reads state.db first, sessions.json fallback
(from #59335)
- slash-commands.md: /model flags --once/--session/--refresh/--provider
+ persist_switch_by_default semantics (from #67832); /reasoning
messaging row gains level list + --global; /history timestamps note
(from #50691)
- environment-variables.md: TERMINAL_DOCKER_ENV,
TERMINAL_DOCKER_EXTRA_ARGS (from #50691); MEM0_MODE/HOST/USER_ID/
AGENT_ID rewritten for the current tri-mode plugin
- cron-script-only.md: interpreter table row matches bash-from-PATH
3334db67a4