> Mux working on behalf of Mike.
## Summary
- add an enabled chat model config lookup by ID for internal callers
- keep `spawn_agent` unchanged while threading an internal model
override through child subagent chat creation
- extend chatd coverage for inherited bindings, plan mode, and internal
override behavior
## Validation
- `go test ./coderd/x/chatd ./coderd/database/dbauthz`
- `make lint`