Follow-up to the mode-switch teardown split: classify legacy secondaries
via an explicit isLegacySecondary() helper, keep open-pane gateway owners
in the boot keep-set, and cover the explicit `local` registry source not
being classified as legacy.
Salvaged from PR #94370. The PR's off-topic edit-composer changes
(user-edit-composer.tsx, user-message-edit.test.tsx — an unrelated edit
submit-cooldown tweak) were dropped from this cherry-pick.
Dropped-files: apps/desktop/src/components/assistant-ui/thread/user-edit-composer.tsx, apps/desktop/src/components/assistant-ui/thread/user-message-edit.test.tsx
joaomarcos
3cf4f7abf7