chore(deps): drop frontend/yarn.lock — repo uses npm everywhere

Both lockfiles were updated on every npm install, creating a drift surface
for nothing. CI, Docker and dev all use npm, so yarn.lock has no consumer.
Add it to .gitignore so future yarn-curious developers don't accidentally
re-introduce it.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

.gitignore

@@ -13,3 +13,7 @@ scripts/large-data.sql
 .vitest-attachments
 **/test-results/
 .worktrees/
+.superpowers/
+
+# Repo uses npm; yarn.lock is ignored to avoid double-lockfile drift.
+frontend/yarn.lock