feat(FN-4441): complete Step 1 — add node_modules cache layer

Fusion-Task-Id: FN-4441
Fusion-Task-Lineage: e3d7d9fe-cd8e-4c00-bb13-0fdb9ce91c34

.github/actions/setup-node-pnpm/action.yml

@@ -30,7 +30,20 @@ runs:
         cache: pnpm
         registry-url: ${{ inputs.registry-url }}
 
-    - name: Install dependencies
+    # Cache node_modules only for the exact pnpm-lock.yaml hash and Node/OS tuple.
+    # Intentionally no restore-keys fallback: partial restores can create inconsistent trees.
+    - name: Cache node_modules
+      id: node-modules-cache
       if: ${{ inputs.skip-install != 'true' }}
+      uses: actions/cache@v4
+      with:
+        path: |
+          node_modules
+          **/node_modules
+          !**/.cache
+        key: node-modules-${{ runner.os }}-node${{ inputs.node-version }}-${{ hashFiles('pnpm-lock.yaml') }}
+
+    - name: Install dependencies
+      if: ${{ inputs.skip-install != 'true' && steps.node-modules-cache.outputs.cache-hit != 'true' }}
       shell: bash
       run: pnpm install ${{ inputs.install-args }}