Update GitHub Actions CI to use pnpm

Update CI workflow to use pnpm instead of yarn for dependency management.

Changes:
- Add pnpm/action-setup@v4 step to install pnpm in both test and lint jobs
- Change cache from 'yarn' to 'pnpm' in setup-node action
- Set pnpm version to 9 for CI stability
- Keep existing make commands (already updated to use pnpm)

The workflow will now:
1. Install pnpm v9
2. Setup Node.js with pnpm caching
3. Run make install (which uses pnpm)
4. Run tests, build, and lint with pnpm

Author: claude

.github/workflows/ci.yml

@@ -19,11 +19,16 @@ jobs:
     - name: Checkout code
       uses: actions/checkout@v4
 
+    - name: Install pnpm
+      uses: pnpm/action-setup@v4
+      with:
+        version: 9
+
     - name: Setup Node.js ${{ matrix.node-version }}
       uses: actions/setup-node@v4
       with:
         node-version: ${{ matrix.node-version }}
-        cache: 'yarn'
+        cache: 'pnpm'
 
     - name: Install dependencies
       run: make install
@@ -50,11 +55,16 @@ jobs:
     - name: Checkout code
       uses: actions/checkout@v4
 
+    - name: Install pnpm
+      uses: pnpm/action-setup@v4
+      with:
+        version: 9
+
     - name: Setup Node.js
       uses: actions/setup-node@v4
       with:
         node-version: 20.x
-        cache: 'yarn'
+        cache: 'pnpm'
 
     - name: Install dependencies
       run: make install