Stop Prettier from walking the runner's pnpm store

The Gitea runner uses a workspace-local .pnpm-store/, so format:check
was scanning 1200+ dependency-cache JSON files. A .prettierignore
keeps Prettier off generated and vendored trees regardless of where
pnpm decides to drop its store.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.prettierignore

@@ -0,0 +1,19 @@
+# Dependency stores
+node_modules/
+.pnpm-store/
+
+# Build output
+dist/
+build/
+
+# Test coverage
+coverage/
+
+# Generated by Prisma
+**/generated/
+
+# Lockfiles (managed by their respective tools)
+pnpm-lock.yaml
+
+# Editor / OS
+.DS_Store