Add guidance to use @strict: true by default in new compiler tests (#1480)

Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: jakebailey <[email protected]>

Author: Copilot

.github/copilot-instructions.md

@@ -56,8 +56,10 @@ export interface Person {
 function greet(person) {
     console.log(`Hello, ${person.name}!`);
 }
-```
-
+```
+
+**New compiler tests should always enable strict mode (`@strict: true`) unless the bug specifically involves non-strict mode behavior.**
+
 Tests don't always need the above `@option`s specified, but they are common to specify or modify.
 Tests can be run with multiple settings for a given option by using a comma-separated list (e.g. `@option: settingA,settingB`).
 `@filename` is only required when a test has multiple files, or when writing a test for a single JavaScript file (where `allowJs` or `checkJs` is enabled).