fix(scripts): allow .prettierignore in coverage diff gate

.prettierignore is a config file with no executable code. Without this
allowlist entry, pnpm coverage:diff fails with no-coverage-data when
.prettierignore is part of the diff.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

scripts/coverage/diff.mjs

@@ -46,6 +46,7 @@ const ALLOWED_GLOBS = [
   /\.json$/,
   /\.ya?ml$/,
   /\.gitignore$/,
+  /\.prettierignore$/,
   /\.npmrc$/,
   /(^|\/)\.env(\.[^/]+)?$/, // .env, .env.example, .env.local, etc.
   // Shell scripts (not Vitest-covered)