A plain /model <name> switch only lasted for the current session — every
new session reverted to the previously-configured model, so users had to
re-switch every time (e.g. glm-5.1 -> glm-5.2 on every launch).
Persist-by-default is now the behavior across all three /model surfaces
(CLI, gateway, TUI/dashboard), gated by a new config key
model.persist_switch_by_default (default true):
/model <name> switch model (persists to config.yaml)
/model <name> --session switch for this session only
/model <name> --global switch and persist (explicit, unchanged)
The effective persistence is resolved once via resolve_persist_behavior()
in hermes_cli/model_switch.py so --session opts out, --global opts in,
and the config-gated default applies otherwise. --global remains a valid
explicit no-op alias for the new default.
