The composer's preset seat stays removed here — this layer moved it to the
hero chip and the session-header action — so only the model seat takes the
`modelSeatLocked` narrowing master introduced.
Conflicts:
apps/web/tests/snapshots/*/*.expected.md
packages/client/connection/README.md
packages/client/connection/README.zh.md
packages/client/ui-conversation/src/client/skeleton/InputBar.tsx
packages/host/apiproxy/src/fetch/client.ts