Add copilot-instructions.md (#3810)

.github/copilot-instructions.md

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+## Making changes
+
+### Tests
+
+Whenever possible, changes should be accompanied by non-trivial tests that meaningfully exercise the core functionality of the new code being introduced.
+
+All tests are in the `Test/` directory at the repo root. Fast unit tests are in the `Test/L0` directory and by convention have the suffix `L0.cs`. For example: unit tests for a hypothetical `src/Runner.Worker/Foo.cs` would go in `src/Test/L0/Worker/FooL0.cs`.
+
+Run tests using this command:
+
+```sh
+cd src && ./dev.sh test
+```
+
+### Formatting
+
+After editing .cs files, always format the code using this command:
+
+```sh
+cd src && ./dev.sh format
+```
+
+### Feature Flags
+
+Wherever possible, all changes should be safeguarded by a feature flag; `Features` are declared in [Constants.cs](src/Runner.Common/Constants.cs).