Add pnpm to main action

Author: ntotten

.github/instructions/workflows.instructions.md

@@ -24,6 +24,7 @@ applyTo: ".github/workflows/*.yaml"
 - Linux tests require Xvfb for display: `/usr/bin/Xvfb :99 -screen 0 1024x768x24`
 - Set `DISPLAY: ":99.0"` environment variable for tests on Linux
 - Tests run via `npm test`
+- Include `pnpm/action-setup@v4` in test jobs - some test fixtures use `packageManager: pnpm` to verify pnpm compatibility
 
 ## Permissions
 

.github/workflows/main.yaml

@@ -28,6 +28,10 @@ jobs:
       - uses: actions/setup-node@v6
         with:
           node-version-file: ".nvmrc"
+      # pnpm is needed for test fixtures that use packageManager: pnpm
+      - uses: pnpm/action-setup@v4
+        with:
+          version: 10
 
       - run: /usr/bin/Xvfb :99 -screen 0 1024x768x24 > /dev/null 2>&1 & echo "Started xvfb"
         shell: bash