feat: append user instructions from file

[namastex888]

This change adds an append_user_instructions_file configuration override so callers can tack on additional guidance without replacing the built‑in system prompt.

Key updates:

• load and resolve the new file path alongside experimental_instructions_file, append its contents to the generated <user_instructions> payload, and expose the resolved path on Config
• allow profiles to opt-in via append_user_instructions_file while preserving existing precedence rules and AGENTS.md behavior
• extend the CLI streaming integration test to assert the appended marker is present in the outbound user instructions message

Testing performed:

• cargo fmt
• cargo test -p codex-core
• cargo run -p codex-cli -- exec --skip-git-repo-check -c "append_user_instructions_file="./wish.md"" "identity?"

This PR contains the same changes as PR #1 in the fork namastexlabs/codex.

[github-actions]

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[namastex888]

I have read the CLA Document and I hereby sign the CLA

[namastex888]

recheck

[etraut-openai]

Thanks for the contribution, and apologies for the slow response. We've received many PRs, and we don't have the bandwidth on the codex team to review all of them.

We've updated our contribution guidelines to clarify that we're currently accepting contributions for bugs and security fixes, but we're not generally accepting new features at this time. We need to make sure that all new features compose well with both existing and upcoming features and fit into our roadmap. If you would like to propose a new feature, please file or upvote an enhancement request in the issue tracker. We will generally prioritize new features based on community feedback.