Standardize shell execution on unified exec (#39772)

## What changed

- Use `exec_command` and `write_stdin` as the shell tool surface.
- Treat legacy `default`, `local`, and `shell_command` model metadata as `unified_exec`.
- Remove obsolete shell-selection configuration and runtime paths while preserving the feature and policy gates for zsh fork execution.

GitOrigin-RevId: d743cbe598630d73052f1fecad680c4cde17977d

codex-rs/core/src/config/config_tests.rs

@@ -6440,8 +6440,6 @@ async fn legacy_toggles_map_to_features() -> std::io::Result<()> {
 
     assert!(config.features.enabled(Feature::UnifiedExec));
 
-    assert!(config.use_experimental_unified_exec_tool);
-
     Ok(())
 }
 
@@ -11033,6 +11031,7 @@ async fn feature_requirements_can_still_disable_unified_exec() -> std::io::Resul
 [features]
 unified_exec = false
 shell_tool = true
+unified_exec_zsh_fork = false
 "#,
             ),
         )
@@ -11041,14 +11040,27 @@ shell_tool = true
 
     assert!(!config.features.enabled(Feature::UnifiedExec));
     assert!(config.features.enabled(Feature::ShellTool));
+    assert!(!config.features.enabled(Feature::UnifiedExecZshFork));
+    assert!(
+        !config
+            .startup_warnings
+            .iter()
+            .any(|warning| warning.contains("Ignoring unknown `features` requirement")),
+        "{:?}",
+        config.startup_warnings
+    );
 
     config
         .features
         .enable(Feature::UnifiedExec)
         .expect("managed feature mutations should normalize successfully");
-
+    config
+        .features
+        .enable(Feature::UnifiedExecZshFork)
+        .expect("managed feature updates should preserve administrator policy");
     assert!(!config.features.enabled(Feature::UnifiedExec));
     assert!(config.features.enabled(Feature::ShellTool));
+    assert!(!config.features.enabled(Feature::UnifiedExecZshFork));
 
     Ok(())
 }

codex-rs/core/src/config/mod.rs

@@ -991,9 +991,6 @@ pub struct Config {
     /// Configuration for the experimental code-mode tool surface.
     pub code_mode: CodeModeConfig,
 
-    /// If set to `true`, used only the experimental unified exec tool.
-    pub use_experimental_unified_exec_tool: bool,
-
     /// Maximum poll window for background terminal output (`write_stdin`), in milliseconds.
     /// Default: `300000` (5 minutes).
     pub background_terminal_max_timeout: u64,
@@ -3774,8 +3771,6 @@ impl Config {
             config
         };
 
-        let use_experimental_unified_exec_tool = features.enabled(Feature::UnifiedExec);
-
         let forced_chatgpt_workspace_id = cfg
             .forced_chatgpt_workspace_id
             .clone()
@@ -4175,7 +4170,6 @@ impl Config {
             update_plan_enabled,
             tool_registry,
             code_mode,
-            use_experimental_unified_exec_tool,
             background_terminal_max_timeout,
             ghost_snapshot,
             multi_agent_v2,