Add `developer_instructions_file` for stronger local guidance

What variant of Codex are you using?

CLI

What feature would you like to see?

I would like to propose a new config key: developer_instructions_file.

This is a practical middle ground between:

• AGENTS.md, which is useful but often has weaker adherence in practice, and
• a hypothetical --append-system-prompt-file, which seems more invasive for Codex CLI.

Additional information

Context and problem

I ran small local experiments and found that:

• Guidance in AGENTS.md is not always followed consistently enough for strict workflows.
• Guidance injected as developer instructions has much better adherence.

I also tried to emulate an "append system prompt file" flow using model_instructions_file, but that creates a maintenance burden:

• I need to track and copy upstream base prompt content (for example from codex-rs/core/prompt.md and related model prompt behavior).
• Any upstream changes to built-in instructions can break or drift my local setup.

Why this over --append-system-prompt-file

--append-system-prompt-file is appealing, but for Codex CLI it seems riskier:

• It touches base/system instruction composition.
• It can interact with model-specific templates and personality logic.

developer_instructions_file fits existing architecture better:

• Lower implementation risk.
• Clearer scope and intent.
• Better adherence than AGENTS.md for strict local policy.

Compatibility

• Fully backward compatible.
• No behavior change unless explicitly configured.
• Relationship with developer_instructions: developer_instructions_file provides a reusable baseline, while developer_instructions remains the inline per-config override.
• If both are set, combine as: file_content + "\n\n" + developer_instructions, so inline text can refine or tighten the baseline file content.