Fix CI: switch to actions/setup-node, purge .yarn/ from git (was causing npm install failure)

- .github/workflows/deploy.yml: replace volta-cli/action with actions/setup-node@v4
  (reads node version from package.json volta.node field), use npm ci for reproducible builds
- .gitignore: replace Yarn Berry rules (.yarn/cache kept tracked) with full .yarn/ exclusion
- git rm: purge .yarn/cache + .yarn/releases/yarn-4.6.0.cjs (old Gatsby Yarn 4 artifacts)
  that were being checked out on CI alongside npm project

Author: Gopal S Akshintala

.github/workflows/deploy.yml

@@ -21,11 +21,14 @@ jobs:
       - name: Checkout
         uses: actions/checkout@v4
 
-      - name: Setup Volta + Node
-        uses: volta-cli/action@v4
+      - name: Setup Node
+        uses: actions/setup-node@v4
+        with:
+          node-version-file: package.json
+          cache: npm
 
       - name: Install dependencies
-        run: npm install
+        run: npm ci
 
       - name: Build with Astro
         run: npm run build

.gitignore

@@ -26,12 +26,5 @@ node_modules/
 npm-debug.log
 yarn-error.log
 
-# Yarn
-.yarn/*
-!.yarn/cache
-!.yarn/patches
-!.yarn/plugins
-!.yarn/releases
-!.yarn/sdks
-!.yarn/versions
-.yarn/*.gz
+# Yarn (not used — project uses npm)
+.yarn/