fix(core): preserve config.toml symlinks on write

[ryoppippi]

Fixes #6646

What

When Codex writes to ~/.codex/config.toml, we persist via a temporary file + rename. If config.toml is a symlink (e.g. managed externally), renaming over it replaces the symlink with a regular file.

This PR follows the symlink chain and atomically writes to the final target file instead, preserving the symlink.

Changes

• Follow config.toml symlink chain and write to the final target path
• Create the temp file in the target directory so the atomic rename works
• Return a clear error when the symlink target is read-only
• Add a unit test that asserts the symlink is preserved and the target is updated

Testing

• nix develop -c cargo test -p codex-core

[github-actions]

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[ryoppippi]

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

[ryoppippi]

recheck

[etraut-openai]

Thanks for the PR. Our contribution guidelines indicate that we're not currently accepting PRs for new features. The linked issue is a feature request, so I would normally close this PR. However, one could argue that this is a bug fix because Codex does follow symlinks for config.toml, so it should arguably handle updates if a symlink is present. This change is also pretty localized (doesn't affect UI or other user-visible behavior).

[etraut-openai]

I've reviewed your PR. I think it's directionally correct, but it has a lot of shortcomings:

• It has hard-coded undocumented constants for symlink depth limits.
• It doesn't detect symlink cycles or have a good fallback behavior when cycles are detected.
• It doesn't handle all cases where config.toml is written.
• It doesn't encapsulate the symlink-following logic into a utility routine.

I've prepared a separate PR that addresses these issues. I welcome you to review and comment on it.

I'm going to close this PR. Thanks again for taking the time to post it.