## 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>
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