Add docs for local development

[ZIJ]

🧠 AI Assistance Disclosure Policy

Important

Inspired by ghostty.
If you used any AI assistance while contributing to Digger, you must disclose it in this PR.

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✅ AI Disclosure Checklist

• I understand that all AI assistance must be disclosed.
• I did not use AI tools in this contribution.
• I used AI tools and have disclosed details below.

Details (if applicable):

codex to do it all

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💡 Notes

• Trivial auto-completions (single words, short phrases) don’t need disclosure.
• Contributors must understand and take responsibility for any AI-assisted code.
• Failure to disclose is considered disrespectful to maintainers and may delay review.