Remove readonly data structures line from Copilot instructions per feedback

Co-authored-by: kraenhansen <1243959+kraenhansen@users.noreply.github.com>

Author: Copilot

.github/copilot-instructions.md

@@ -68,7 +68,6 @@ npm run bootstrap           # Build native components (weak-node-api, examples)
 - Use structured error handling with custom error types when needed
 
 **Immutable Data Patterns**:
-- Prefer readonly data structures where applicable
 - Use `const` assertions for compile-time immutability
 
 ### Babel Transformation