fix(core): load custom prompts from symlinked Markdown files

[gabrimatic]

• Discover prompts via fs::metadata to follow symlinks

• Add Unix-only symlink test in custom_prompts.rs

• Update docs/prompts.md to mention symlinks

Fixes #3637

[github-actions]

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[gabrimatic]

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

[dedrisian-oai]

@codex pls format and fix clippy warnings if any

[chatgpt-codex-connector]

Note

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[dedrisian-oai]

@gabrimatic Salam! Can you please run just fmt on the PR? Thanks!

[gabrimatic]

@codex pls format and fix clippy warnings if any

[chatgpt-codex-connector]

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[etraut-openai]

Thanks for your contribution, and apologies for taking so long to review. We received a lot of PRs, and we're just now getting to the backlog.

If you're still interested in pursuing this PR, could you please resolve the merge conflicts?

[gabrimatic]

@etraut-openai Sure, I just fixed the conflicts.

[etraut-openai]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. What shall we delve into next?

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