Flip the resolve_persist_behavior() fallback from persist-to-config to
session-only. A plain /model <name> (typed or via any picker — CLI,
TUI/Desktop, gateway) now affects only the current session; --global
persists explicitly, and model.persist_switch_by_default: true restores
the old opt-out behavior for users who want switches to stick.
This is the root cause behind the recurring 'session switch applied
globally' bug class (#61458, #63083, #58290, #61190): every surface
funnels its no-flag default through this one function, so per-surface
patches kept missing paths. Fixing the default fixes all surfaces at
once: CLI typed + picker, TUI/Desktop config.set + slash, gateway typed
+ inline picker.
Builds on liuhao1024's #58371 (--provider session scoping, cherry-picked
as the previous commit) and supersedes the per-surface #61488.
Teknium
8b6fde3a35