[codex] Remove experimental instructions file config (#22724)

## Summary

Remove the deprecated `experimental_instructions_file` config setting
from the typed config surface and the remaining deprecation-notice
plumbing. `model_instructions_file` remains the supported setting and
its loading path is unchanged.

The setting was deprecated when it was renamed to
`model_instructions_file` on January 20, 2026 in
https://github.com/openai/codex/pull/9555.

## Changes

- Remove `experimental_instructions_file` from `ConfigToml` and
`ConfigProfile`.
- Delete the custom config-layer scan and session deprecation notice for
the removed setting.
- Stop clearing the removed field from generated session config locks.
- Remove the obsolete deprecation-notice test case while keeping
`model_instructions_file` coverage intact.

## Validation

- `just write-config-schema`
- `just fmt`
- `cargo test -p codex-config`
- `cargo test -p codex-core model_instructions_file`
- `just fix -p codex-core`
- `git diff --check`

Co-authored-by: Codex <noreply@openai.com>

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

@@ -1,23 +1,16 @@
 #![cfg(not(target_os = "windows"))]
 
 use anyhow::Ok;
-use codex_app_server_protocol::ConfigLayerSource;
-use codex_config::ConfigLayerEntry;
-use codex_config::ConfigLayerStack;
-use codex_config::ConfigRequirements;
-use codex_config::ConfigRequirementsToml;
 use codex_features::Feature;
 use codex_protocol::protocol::DeprecationNoticeEvent;
 use codex_protocol::protocol::EventMsg;
 use core_test_support::responses::start_mock_server;
 use core_test_support::skip_if_no_network;
-use core_test_support::test_absolute_path;
 use core_test_support::test_codex::TestCodex;
 use core_test_support::test_codex::test_codex;
 use core_test_support::wait_for_event_match;
 use pretty_assertions::assert_eq;
 use std::collections::BTreeMap;
-use toml::Value as TomlValue;
 
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
 async fn emits_deprecation_notice_for_legacy_feature_flag() -> anyhow::Result<()> {
@@ -60,62 +53,6 @@ async fn emits_deprecation_notice_for_legacy_feature_flag() -> anyhow::Result<()
     Ok(())
 }
 
-#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
-async fn emits_deprecation_notice_for_experimental_instructions_file() -> anyhow::Result<()> {
-    skip_if_no_network!(Ok(()));
-
-    let server = start_mock_server().await;
-
-    let mut builder = test_codex().with_config(|config| {
-        let mut table = toml::map::Map::new();
-        table.insert(
-            "experimental_instructions_file".to_string(),
-            TomlValue::String("legacy.md".to_string()),
-        );
-        let config_layer = ConfigLayerEntry::new(
-            ConfigLayerSource::User {
-                file: test_absolute_path("/tmp/config.toml"),
-                profile: None,
-            },
-            TomlValue::Table(table),
-        );
-        let config_layer_stack = ConfigLayerStack::new(
-            vec![config_layer],
-            ConfigRequirements::default(),
-            ConfigRequirementsToml::default(),
-        )
-        .expect("build config layer stack");
-        config.config_layer_stack = config_layer_stack;
-    });
-
-    let TestCodex { codex, .. } = builder.build(&server).await?;
-
-    let notice = wait_for_event_match(&codex, |event| match event {
-        EventMsg::DeprecationNotice(ev)
-            if ev.summary.contains("experimental_instructions_file") =>
-        {
-            Some(ev.clone())
-        }
-        _ => None,
-    })
-    .await;
-
-    let DeprecationNoticeEvent { summary, details } = notice;
-    assert_eq!(
-        summary,
-        "`experimental_instructions_file` is deprecated and ignored. Use `model_instructions_file` instead."
-            .to_string(),
-    );
-    assert_eq!(
-        details.as_deref(),
-        Some(
-            "Move the setting to `model_instructions_file` in config.toml (or under a profile) to load instructions from a file."
-        ),
-    );
-
-    Ok(())
-}
-
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
 async fn emits_deprecation_notice_for_web_search_feature_flag_values() -> anyhow::Result<()> {
     skip_if_no_network!(Ok(()));