ci: add pnpm auto-detection to shared plugin CI workflow

When pnpm-lock.yaml is present, use pnpm for install, lint, type-check,
format check, tests, and security audit instead of npm. Repos using npm
are unaffected (falls back to existing npm behavior).

This fixes the npm/pnpm inconsistency in headlamp-polaris-plugin where
local development uses pnpm but CI used npm, causing:
- Different transitive dependency resolution (TypeScript not hoisted)
- Different audit results (pnpm audit vs npm audit)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.github/workflows/plugin-ci.yaml

@@ -18,29 +18,74 @@ jobs:
       - name: Checkout
         uses: actions/checkout@v6
 
+      - name: Detect package manager
+        id: pkg-manager
+        run: |
+          if [ -f "pnpm-lock.yaml" ]; then
+            echo "manager=pnpm" >> $GITHUB_OUTPUT
+          else
+            echo "manager=npm" >> $GITHUB_OUTPUT
+          fi
+
+      - name: Setup pnpm
+        if: steps.pkg-manager.outputs.manager == 'pnpm'
+        uses: pnpm/action-setup@v4
+        with:
+          run_install: false
+
       - name: Setup Node
         uses: actions/setup-node@v4
         with:
           node-version: ${{ inputs.node-version }}
-          cache: 'npm'
+          cache: ${{ steps.pkg-manager.outputs.manager }}
 
       - name: Install dependencies
-        run: npm ci
+        run: |
+          if [ "${{ steps.pkg-manager.outputs.manager }}" = "pnpm" ]; then
+            pnpm install --frozen-lockfile
+          else
+            npm ci
+          fi
 
       - name: Build plugin
         run: npx @kinvolk/headlamp-plugin build
 
       - name: Lint
-        run: npm run lint
+        run: |
+          if [ "${{ steps.pkg-manager.outputs.manager }}" = "pnpm" ]; then
+            pnpm run lint
+          else
+            npm run lint
+          fi
 
       - name: Type-check
-        run: npm run tsc
+        run: |
+          if [ "${{ steps.pkg-manager.outputs.manager }}" = "pnpm" ]; then
+            pnpm run tsc
+          else
+            npm run tsc
+          fi
 
       - name: Format check
-        run: npm run format:check
+        run: |
+          if [ "${{ steps.pkg-manager.outputs.manager }}" = "pnpm" ]; then
+            pnpm run format:check
+          else
+            npm run format:check
+          fi
 
       - name: Run tests
-        run: npm test
+        run: |
+          if [ "${{ steps.pkg-manager.outputs.manager }}" = "pnpm" ]; then
+            pnpm test
+          else
+            npm test
+          fi
 
       - name: Security audit
-        run: npm audit --omit=dev
+        run: |
+          if [ "${{ steps.pkg-manager.outputs.manager }}" = "pnpm" ]; then
+            pnpm audit --prod
+          else
+            npm audit --omit=dev
+          fi