Downgrade Knip 6 to 5 for CI compatibility

Knip 6 uses oxc-parser which attempts a 6GB ArrayBuffer allocation
that fails on the CI runner (3.7GB RAM, no swap). This is a known
oxc allocator issue (oxc-project/oxc#20513) with no fix yet.
Revert to Knip 5 which uses TypeScript's parser. Also revert the
NODE_OPTIONS workaround since it's no longer needed.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.gitea/workflows/ci.yml

@@ -20,7 +20,7 @@ jobs:
           node-version: 22
 
       - run: pnpm install --frozen-lockfile
-      - run: NODE_OPTIONS="--max-old-space-size=2048" pnpm check
+      - run: pnpm check
 
   build-image:
     if: startsWith(github.ref, 'refs/tags/')