[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/session/config_lock.rs

@@ -186,7 +186,6 @@ fn drop_lockfile_inputs(lock_config: &mut ConfigToml) {
     lock_config.profiles.clear();
     clear_config_lock_debug_controls(lock_config);
     lock_config.model_instructions_file = None;
-    lock_config.experimental_instructions_file = None;
     lock_config.experimental_compact_prompt_file = None;
     lock_config.model_catalog_json = None;
     lock_config.sandbox_mode = None;

codex-rs/core/src/session/session.rs

@@ -564,19 +564,6 @@ impl Session {
                     }),
                 });
             }
-            if crate::config::uses_deprecated_instructions_file(&config.config_layer_stack) {
-                post_session_configured_events.push(Event {
-                    id: INITIAL_SUBMIT_ID.to_owned(),
-                    msg: EventMsg::DeprecationNotice(DeprecationNoticeEvent {
-                        summary: "`experimental_instructions_file` is deprecated and ignored. Use `model_instructions_file` instead."
-                            .to_string(),
-                        details: Some(
-                            "Move the setting to `model_instructions_file` in config.toml (or under a profile) to load instructions from a file."
-                                .to_string(),
-                        ),
-                    }),
-                });
-            }
             for message in &config.startup_warnings {
                 post_session_configured_events.push(Event {
                     id: "".to_owned(),