## Why
`TurnEnvironmentSelections::new` rewrote the primary environment's
explicit `cwd` to the legacy fallback cwd. For a remote-first selection,
this could replace the remote working directory with a local fallback
path and made the legacy cwd overlay authoritative over
environment-owned state.
## What changed
- Preserve every explicit environment cwd when constructing turn
environment selections.
- Keep `cwd`-only app-server updates compatible by rebuilding the
default environment selections at the requested cwd.
- Cover both explicit primary cwd preservation and cwd-only updates
reaching the model-visible execution environment.
## Testing
- `just test -p codex-core
session_update_settings_does_not_rewrite_sticky_environment_cwds`
- `just test -p codex-core
environment_settings_preserve_explicit_primary_cwd`
- `just test -p codex-app-server
thread_settings_update_cwd_retargets_default_environment`