feat: rename experimental_instructions_file to model_instructions_file (#9555)

A user who has `experimental_instructions_file` set will now see this:

<img width="888" height="660" alt="image"
src="https://github.com/user-attachments/assets/51c98312-eb9b-4881-81f1-bea6677e158d"
/>

And a `codex exec` would include this warning:

<img width="888" height="660" alt="image"
src="https://github.com/user-attachments/assets/a89f62be-1edf-4593-a75e-e0b4a762ed7d"
/>

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

@@ -656,7 +656,7 @@ mod unit_tests {
         let contents = r#"
 # This is a field recognized by config.toml that is an AbsolutePathBuf in
 # the ConfigToml struct.
-experimental_instructions_file = "./some_file.md"
+model_instructions_file = "./some_file.md"
 
 # This is a field recognized by config.toml.
 model = "gpt-1000"
@@ -669,7 +669,7 @@ foo = "xyzzy"
         let normalized_toml_value = resolve_relative_paths_in_config_toml(user_config, base_dir)?;
         let mut expected_toml_value = toml::map::Map::new();
         expected_toml_value.insert(
-            "experimental_instructions_file".to_string(),
+            "model_instructions_file".to_string(),
             TomlValue::String(
                 AbsolutePathBuf::resolve_path_against_base("./some_file.md", base_dir)?
                     .as_path()