Exclude .pnpm-store from prettier and git

On the Gitea runner, pnpm's content-addressable store ends up inside
the workspace (.pnpm-store/) instead of the global cache location -
format:check was scanning its content-addressable blobs as if they
were source files, some of which happen to parse as JS/TS-like text
and crash prettier's parser outright rather than just wasting time.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.prettierignore

@@ -1,2 +1,7 @@
 # Machine-generated - pnpm owns this file's formatting, not prettier.
 pnpm-lock.yaml
+
+# pnpm's local content-addressable store - on some runners this ends up inside the workspace
+# instead of the global cache location; its blobs aren't source files (some happen to parse as
+# JS/TS-like content, which crashes prettier's parser rather than just wasting time on them).
+.pnpm-store/