## Summary
Add a new app-server `marketplace/remove` RPC on top of the shared
marketplace-remove implementation.
This change:
- adds `MarketplaceRemoveParams` / `MarketplaceRemoveResponse` to the
app-server protocol
- wires the new request through `codex_message_processor`
- reuses the shared core marketplace-remove flow from the stacked
refactor PR
- updates generated schema files and adds focused app-server coverage
## Validation
- `just write-app-server-schema`
- `just fmt`
- heavy compile/test coverage deferred to GitHub CI per request
