chore(core) Remove Feature::PowershellUtf8 (#15128)

## Summary
This feature has been enabled for powershell for a while now, let's get
rid of the logic

## Testing
- [x] Unit tests

codex-rs/core/config.schema.json

@@ -431,9 +431,6 @@
             "plugins": {
               "type": "boolean"
             },
-            "powershell_utf8": {
-              "type": "boolean"
-            },
             "prevent_idle_sleep": {
               "type": "boolean"
             },
@@ -2040,9 +2037,6 @@
         "plugins": {
           "type": "boolean"
         },
-        "powershell_utf8": {
-          "type": "boolean"
-        },
         "prevent_idle_sleep": {
           "type": "boolean"
         },

codex-rs/core/src/tools/runtimes/shell.rs

@@ -33,7 +33,6 @@ use crate::tools::sandboxing::ToolError;
 use crate::tools::sandboxing::ToolRuntime;
 use crate::tools::sandboxing::sandbox_override_for_first_attempt;
 use crate::tools::sandboxing::with_cached_approval;
-use codex_features::Feature;
 use codex_network_proxy::NetworkProxy;
 use codex_protocol::models::PermissionProfile;
 use codex_protocol::protocol::ReviewDecision;
@@ -227,9 +226,7 @@ impl ToolRuntime<ShellRequest, ExecToolCallOutput> for ShellRuntime {
             &req.cwd,
             &req.explicit_env_overrides,
         );
-        let command = if matches!(session_shell.shell_type, ShellType::PowerShell)
-            && ctx.session.features().enabled(Feature::PowershellUtf8)
-        {
+        let command = if matches!(session_shell.shell_type, ShellType::PowerShell) {
             prefix_powershell_script_with_utf8(&command)
         } else {
             command

codex-rs/core/src/tools/runtimes/unified_exec.rs

@@ -36,7 +36,6 @@ use crate::unified_exec::NoopSpawnLifecycle;
 use crate::unified_exec::UnifiedExecError;
 use crate::unified_exec::UnifiedExecProcess;
 use crate::unified_exec::UnifiedExecProcessManager;
-use codex_features::Feature;
 use codex_network_proxy::NetworkProxy;
 use codex_protocol::models::PermissionProfile;
 use codex_protocol::protocol::ReviewDecision;
@@ -200,9 +199,7 @@ impl<'a> ToolRuntime<UnifiedExecRequest, UnifiedExecProcess> for UnifiedExecRunt
             &req.cwd,
             &req.explicit_env_overrides,
         );
-        let command = if matches!(session_shell.shell_type, ShellType::PowerShell)
-            && ctx.session.features().enabled(Feature::PowershellUtf8)
-        {
+        let command = if matches!(session_shell.shell_type, ShellType::PowerShell) {
             prefix_powershell_script_with_utf8(&command)
         } else {
             command

codex-rs/core/tests/suite/shell_command.rs

@@ -1,7 +1,6 @@
 use std::time::Duration;
 
 use anyhow::Result;
-use codex_features::Feature;
 use core_test_support::assert_regex_match;
 use core_test_support::responses::ev_assistant_message;
 use core_test_support::responses::ev_completed;
@@ -251,16 +250,7 @@ async fn shell_command_times_out_with_timeout_ms() -> anyhow::Result<()> {
 async fn unicode_output(login: bool) -> anyhow::Result<()> {
     skip_if_no_network!(Ok(()));
 
-    #[allow(clippy::expect_used)]
-    let harness = shell_command_harness_with(|builder| {
-        builder.with_model("gpt-5.2").with_config(|config| {
-            config
-                .features
-                .enable(Feature::PowershellUtf8)
-                .expect("test config should allow feature update");
-        })
-    })
-    .await?;
+    let harness = shell_command_harness_with(|builder| builder.with_model("gpt-5.2")).await?;
 
     // We use a child process on windows instead of a direct builtin like 'echo' to ensure that Powershell
     // config is actually being set correctly.
@@ -286,16 +276,7 @@ async fn unicode_output(login: bool) -> anyhow::Result<()> {
 async fn unicode_output_with_newlines(login: bool) -> anyhow::Result<()> {
     skip_if_no_network!(Ok(()));
 
-    #[allow(clippy::expect_used)]
-    let harness = shell_command_harness_with(|builder| {
-        builder.with_model("gpt-5.2").with_config(|config| {
-            config
-                .features
-                .enable(Feature::PowershellUtf8)
-                .expect("test config should allow feature update");
-        })
-    })
-    .await?;
+    let harness = shell_command_harness_with(|builder| builder.with_model("gpt-5.2")).await?;
 
     let call_id = "unicode_output";
     mount_shell_responses_with_timeout(

codex-rs/features/src/lib.rs

@@ -132,8 +132,6 @@ pub enum Feature {
     ChildAgentsMd,
     /// Allow the model to request `detail: "original"` image outputs on supported models.
     ImageDetailOriginal,
-    /// Enforce UTF8 output in Powershell.
-    PowershellUtf8,
     /// Compress request bodies (zstd) when sending streaming requests to codex-backend.
     EnableRequestCompression,
     /// Enable collab tools.
@@ -689,18 +687,6 @@ pub const FEATURES: &[FeatureSpec] = &[
         stage: Stage::Removed,
         default_enabled: false,
     },
-    FeatureSpec {
-        id: Feature::PowershellUtf8,
-        key: "powershell_utf8",
-        #[cfg(windows)]
-        stage: Stage::Stable,
-        #[cfg(windows)]
-        default_enabled: true,
-        #[cfg(not(windows))]
-        stage: Stage::UnderDevelopment,
-        #[cfg(not(windows))]
-        default_enabled: false,
-    },
     FeatureSpec {
         id: Feature::EnableRequestCompression,
         key: "enable_request_compression",