chore: add knip to CI pipeline

Adds the `pnpm run knip` command to the CI pipeline to detect unused files, dependencies, and exports, as recommended in the code review.

Author: google-labs-jules[bot]

.github/workflows/ci.yml

@@ -39,6 +39,9 @@ jobs:
       - name: Type check
         run: pnpm exec tsc --noEmit
 
+      - name: Knip
+        run: pnpm run knip
+
       - name: Format check
         run: pnpm run format:check