Add copilot-instructions.md file

[mikachan]

What?

Adds a .github/copilot-instructions.md file to guide GitHub Copilot's PR review behaviour repo-wide.

The file is split into three short sections:

• PR review guidelines: how Copilot should review (short comments, no top-level summary, no style nits as these are caught by lint and PHPCS).
• What not to flag: calibration rules to suppress recurring false positives (hallucinated APIs, refactor requests for inherited duplication, theoretical edge cases).
• Gutenberg-specific context: small set of guardrails against Copilot second-guessing intentional WordPress patterns (@wordpress/data, __()/_x()/_n(), the block-editor → editor → edit-post package layering rule).

Why?

Copilot's default PR review in this repo tends toward long summary restatement and style observations that lint already covers, while occasionally producing comments based on hallucinated or unverified WordPress core APIs.

I sampled 50 recent Copilot reviews in this repo to form these rules:

• Already strong (so don't duplicate): cross-file consistency, hook deps, duplication detection, doc/code drift, numeric edge cases.
• Recurring pushback themes from PR authors: hallucinated WordPress core APIs / strings, requests to refactor pre-existing inherited duplication, over-cautious "what about this edge case" comments with no real-world impact.

How?

GitHub Copilot reads .github/copilot-instructions.md and merges its contents into every PR review in the repo.

If this works well and folks agree, we could extend this to include further instruction files for more specific things, like code changes in specific packages having an additional set of guidance. I'm not sure how far we need to take this for it to be as helpful as possible.

Testing Instructions

Let's see if this improves the quality of Copilot's reviews once it's merged.

Use of AI Tools

I used Claude Code to sample the recent Copilot reviews and draft the instructions.

[aduth]

Let's see if this improves the quality of Copilot's reviews once it's merged.

Would it apply if we asked for Copilot's review here? 😄 (Maybe with some sample code changes if needed)

I'm particularly interested in the point about "NEVER produce a review body". Will it abide that? Is it actually desirable? I mean, if a human would have reasons to leave a top-level review comment, why wouldn't the same apply to an AI agent?

[github-actions]

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Co-authored-by: mikachan <mikachan@git.wordpress.org>
Co-authored-by: aduth <aduth@git.wordpress.org>

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