[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/config/src/config_toml.rs

@@ -485,10 +485,6 @@ pub struct ConfigToml {
     /// See [`crate::types::Notice`] for more details
     pub notice: Option<Notice>,
 
-    /// Legacy, now use features
-    /// Deprecated: ignored. Use `model_instructions_file`.
-    #[schemars(skip)]
-    pub experimental_instructions_file: Option<AbsolutePathBuf>,
     pub experimental_compact_prompt_file: Option<AbsolutePathBuf>,
     pub experimental_use_unified_exec_tool: Option<bool>,
     /// Preferred OSS provider for local models, e.g. "lmstudio" or "ollama".

codex-rs/config/src/profile_toml.rs

@@ -50,9 +50,6 @@ pub struct ConfigProfile {
     pub js_repl_node_module_dirs: Option<Vec<AbsolutePathBuf>>,
     /// Optional absolute path to patched zsh used by zsh-exec-bridge-backed shell execution.
     pub zsh_path: Option<AbsolutePathBuf>,
-    /// Deprecated: ignored. Use `model_instructions_file`.
-    #[schemars(skip)]
-    pub experimental_instructions_file: Option<AbsolutePathBuf>,
     pub experimental_compact_prompt_file: Option<AbsolutePathBuf>,
     pub include_permissions_instructions: Option<bool>,
     pub include_apps_instructions: Option<bool>,