[codex] Honor remote environment cwd and shell

[anp-oai]

Why

Cross-OS remote execution needs both the selected environment cwd and its native shell to survive orchestration. After turn environment selections carry cwd as PathUri, the remaining behavior still rewrites a remote primary cwd to the local legacy fallback and builds commands with the orchestrator session shell. On a Linux-to-Windows connection that prevents a real Windows process from starting.

Stacked on #28122.

What

• only synchronize legacy_fallback_cwd onto a primary local environment
• prefer the selected environment's discovered shell for unified exec, falling back to the session shell when discovery is unavailable
• extend the hermetic Wine test to execute Windows PowerShell in C:\windows and verify successful process lifecycle completion

Testing

• bazel test //codex-rs/core/tests/remote_env_windows:smoke-test --test_output=errors
• just test -p codex-protocol
• just test -p codex-core unified_exec::tests
• just test -p codex-core cwd_update_rewrites_sticky_environment_cwd