chore: add copilot instructions for code reviews (#9676)

Author: maribethb

.github/copilot-instructions.md

@@ -0,0 +1,38 @@
+# Code review guidelines
+
+## General principles
+- **Style:** Suggest `npm run format` or `npm run lint:fix` for formatting issues; do not comment on individual style nits.
+- **Patterns:** Enforce existing Blockly patterns and official docs over new conventions.
+- **Documentation:** Prefer linking to [Blockly Dev Docs](https://developers.google.com/blockly) over duplicating content in comments.
+- **TSDoc:** Public APIs require TSDoc for behavior, params, and returns. Do not include implementation details or historical context unless essential.
+
+## Localization
+- All user-visible strings must use `Blockly.Msg`.
+- New strings must be added to `msg/messages.js`, `msg/json/qqq.json`, and `msg/json/en.json`.
+- Link [this guide](https://developers.google.com/blockly/guides/contribute/core/add_localization_token) if strings are missing or misplaced.
+- PRs that attempt to add translations for non-English strings should be redirected to TranslateWiki via the ([translation guide](hhttps://developers.google.com/blockly/guides/contribute/core/translating)).
+
+## Breaking changes
+### Policy
+- A breaking change is any non-backwards-compatible change to public APIs, behavior, UI, or browser requirements.
+- **Avoid:** Prefer deprecation with migration paths over removal. 
+- **Compatibility:** Must support Safari 15.4+, latest Chrome, and latest Firefox.
+- **Identification:** Flag breaking changes unless all of the following are true:
+  1. PR description explicitly notes it.
+  2. Commit type includes `!` (e.g., `feat!:`).
+  3. Target branch is not `main`.
+
+### Breaking
+- Removing/renaming public methods, properties, or classes.
+- Changing signatures or behavior of existing public methods.
+- Adding required methods to public interfaces.
+- New keyboard shortcuts or context menu items (potential developer conflicts).
+- DOM restructures affecting external CSS/JS.
+- Changes to build output/consumption (e.g., ESM-only).
+- Changes that affect the output of serialization.
+
+### Non-breaking (do not flag)
+- Additive changes (new methods/properties).
+- Internal refactoring (including items marked `@internal`).
+- Tooling/workflow changes.
+- Changes to unreleased code (non-`main` feature branches).