core: add a feature to disable the shell tool (#6481)

`--disable shell_tool` disables the built-in shell tool. This is useful
for MCP-only operation.

---------

Co-authored-by: Michael Bolin <mbolin@openai.com>

codex-rs/core/src/features.rs

@@ -46,6 +46,8 @@ pub enum Feature {
     GhostCommit,
     /// Enable Windows sandbox (restricted token) on Windows.
     WindowsSandbox,
+    /// Enable the default shell tool.
+    ShellTool,
 }
 
 impl Feature {
@@ -301,4 +303,10 @@ pub const FEATURES: &[FeatureSpec] = &[
         stage: Stage::Experimental,
         default_enabled: false,
     },
+    FeatureSpec {
+        id: Feature::ShellTool,
+        key: "shell_tool",
+        stage: Stage::Stable,
+        default_enabled: true,
+    },
 ];