Add code_mode_only feature (#14617)

Summary
- add the code_mode_only feature flag/config schema and wire its
dependency on code_mode
- update code mode tool descriptions to list nested tools with detailed
headers
- restrict available tools for prompt and exec descriptions when
code_mode_only is enabled and test the behavior

Testing
- Not run (not requested)

codex-rs/core/src/features_tests.rs

@@ -58,6 +58,16 @@ fn js_repl_is_experimental_and_user_toggleable() {
     assert_eq!(Feature::JsRepl.default_enabled(), false);
 }
 
+#[test]
+fn code_mode_only_requires_code_mode() {
+    let mut features = Features::with_defaults();
+    features.enable(Feature::CodeModeOnly);
+    features.normalize_dependencies();
+
+    assert_eq!(features.enabled(Feature::CodeModeOnly), true);
+    assert_eq!(features.enabled(Feature::CodeMode), true);
+}
+
 #[test]
 fn guardian_approval_is_experimental_and_user_toggleable() {
     let spec = Feature::GuardianApproval.info();