Replaces the FlatCompat shim (which loaded .eslintrc.cjs via @eslint/eslintrc
and emitted deprecation warnings) with a native ESLint 9 flat config. Adds
typescript-eslint meta-package as the flat-config entrypoint and pins
@eslint/js to v9 to satisfy the eslint@9 peer range. Rule set is preserved
verbatim from the Phase 1A-1 plan (Task 4).