Feat: appServer.requirementList for requirement.toml (#8800)

### Summary
We are exposing requirements via `requirement/list` method from
app-server so that we can conditionally disable the agent mode dropdown
selection in VSCE and correctly setting the default value.

### Sample output
#### `etc/codex/requirements.toml`
<img width="497" height="49" alt="Screenshot 2026-01-06 at 11 32 06 PM"
src="https://github.com/user-attachments/assets/fbd9402e-515f-4b9e-a158-2abb23e866a0"
/>

#### App server response
<img width="1107" height="79" alt="Screenshot 2026-01-06 at 11 30 18 PM"
src="https://github.com/user-attachments/assets/c0d669cd-54ef-4789-a26c-adb2c41950af"
/>

codex-rs/core/src/skills/loader.rs

@@ -410,6 +410,7 @@ mod tests {
     use crate::config_loader::ConfigLayerEntry;
     use crate::config_loader::ConfigLayerStack;
     use crate::config_loader::ConfigRequirements;
+    use crate::config_loader::ConfigRequirementsToml;
     use codex_protocol::protocol::SkillScope;
     use codex_utils_absolute_path::AbsolutePathBuf;
     use pretty_assertions::assert_eq;
@@ -471,7 +472,11 @@ mod tests {
                 TomlValue::Table(toml::map::Map::new()),
             ),
         ];
-        let stack = ConfigLayerStack::new(layers, ConfigRequirements::default())?;
+        let stack = ConfigLayerStack::new(
+            layers,
+            ConfigRequirements::default(),
+            ConfigRequirementsToml::default(),
+        )?;
 
         let got = skill_roots_from_layer_stack(&stack)
             .into_iter()