fix: install shellcheck in PR validation workflow

The shellcheck step fails with "command not found" because shellcheck
is not installed on the runner. Install it from GitHub releases, same
pattern as the actionlint install step.

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

.github/workflows/pr-validation.yaml

@@ -24,6 +24,13 @@ jobs:
       - name: Validate workflow YAML with actionlint
         run: actionlint -color .github/workflows/*.yaml
 
+      - name: Install shellcheck
+        run: |
+          SHELLCHECK_VERSION="0.10.0"
+          mkdir -p "$HOME/.local/bin"
+          curl -fsSL "https://github.com/koalaman/shellcheck/releases/download/v${SHELLCHECK_VERSION}/shellcheck-v${SHELLCHECK_VERSION}.linux.x86_64.tar.xz" \
+            | tar -xJ --strip-components=1 -C "$HOME/.local/bin" "shellcheck-v${SHELLCHECK_VERSION}/shellcheck"
+
       - name: Shellcheck scripts
         run: |
           if ls .github/scripts/*.sh 1>/dev/null 2>&1; then