ci(lotus): gitignore .pnpm-store so knip doesn't scan the runner's store

Run #1858 got past install, tsc and oxlint; knip then failed with 1188
"unused files" — all under .pnpm-store/, because the Gitea runner keeps
pnpm's content store inside the checkout, and it now contains the
from-source matrix-js-sdk tree. knip honours .gitignore, so ignoring the
store directory (verified locally with a synthetic .pnpm-store) is enough.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

.gitignore

@@ -38,3 +38,4 @@ yarn-error.log
 
 *storybook.log
 storybook-static
+.pnpm-store/