[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/src/config/mod.rs

@@ -3407,13 +3407,6 @@ impl Config {
     }
 }
 
-pub(crate) fn uses_deprecated_instructions_file(config_layer_stack: &ConfigLayerStack) -> bool {
-    config_layer_stack
-        .layers_high_to_low()
-        .into_iter()
-        .any(|layer| toml_uses_deprecated_instructions_file(&layer.config))
-}
-
 fn guardian_policy_config_from_requirements(
     requirements_toml: &ConfigRequirementsToml,
 ) -> Option<String> {
@@ -3427,23 +3420,6 @@ fn normalize_guardian_policy_config(value: Option<&str>) -> Option<String> {
     })
 }
 
-fn toml_uses_deprecated_instructions_file(value: &TomlValue) -> bool {
-    let Some(table) = value.as_table() else {
-        return false;
-    };
-    if table.contains_key("experimental_instructions_file") {
-        return true;
-    }
-    let Some(profiles) = table.get("profiles").and_then(TomlValue::as_table) else {
-        return false;
-    };
-    profiles.values().any(|profile| {
-        profile.as_table().is_some_and(|profile_table| {
-            profile_table.contains_key("experimental_instructions_file")
-        })
-    })
-}
-
 /// Returns the path to the Codex configuration directory, which can be
 /// specified by the `CODEX_HOME` environment variable. If not set, defaults to
 /// `~/.codex`.