ci: add pnpm detection to plugin-release workflow

Mirrors the pnpm-detection logic from plugin-ci.yaml. When a repo has
pnpm-lock.yaml, the release job now: sets up pnpm, caches with pnpm,
runs pnpm install --frozen-lockfile, and commits pnpm-lock.yaml (not
package-lock.json) in the release branch.

Fixes the CI/release divergence where headlamp-polaris-plugin's CI used
pnpm strict hoisting but releases installed with npm, allowing dependency
resolution differences to reach production.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

.github/workflows/plugin-release.yaml

@@ -97,11 +97,29 @@ jobs:
         with:
           fetch-depth: 0
 
+      - name: Detect package manager
+        id: pkg-manager
+        run: |
+          if [ -f "pnpm-lock.yaml" ]; then
+            echo "manager=pnpm" >> $GITHUB_OUTPUT
+            echo "lockfile=pnpm-lock.yaml" >> $GITHUB_OUTPUT
+          else
+            echo "manager=npm" >> $GITHUB_OUTPUT
+            echo "lockfile=package-lock.json" >> $GITHUB_OUTPUT
+          fi
+
+      - name: Setup pnpm
+        if: steps.pkg-manager.outputs.manager == 'pnpm'
+        uses: pnpm/action-setup@v4
+        with:
+          run_install: false
+          version: latest
+
       - name: Setup Node
         uses: actions/setup-node@v4
         with:
           node-version: ${{ inputs.node-version }}
-          cache: 'npm'
+          cache: ${{ steps.pkg-manager.outputs.manager }}
 
       - name: Configure Git
         run: |
@@ -110,7 +128,12 @@ jobs:
           git config --global --add safe.directory "$GITHUB_WORKSPACE"
 
       - name: Update version in package.json
-        run: npm version ${{ inputs.version }} --no-git-tag-version --allow-same-version
+        run: |
+          if [ "${{ steps.pkg-manager.outputs.manager }}" = "pnpm" ]; then
+            pnpm version ${{ inputs.version }} --no-git-tag-version --allow-same-version
+          else
+            npm version ${{ inputs.version }} --no-git-tag-version --allow-same-version
+          fi
 
       - name: Update artifacthub-pkg.yml
         run: |
@@ -132,7 +155,12 @@ jobs:
           fi
 
       - 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
@@ -175,7 +203,7 @@ jobs:
           VERSION="${{ inputs.version }}"
           BRANCH="release/v${VERSION}"
           git checkout -b "$BRANCH"
-          git add package.json package-lock.json artifacthub-pkg.yml
+          git add package.json "${{ steps.pkg-manager.outputs.lockfile }}" artifacthub-pkg.yml
           git commit -m "release: v${VERSION}"
           git tag "v${VERSION}"
           git push origin "$BRANCH" --tags