ci: add GitHub Actions CI (shellcheck, syntax, preview)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

.github/workflows/ci.yml

@@ -0,0 +1,63 @@
+name: CI
+
+on:
+  push:
+  pull_request:
+
+jobs:
+  shellcheck:
+    name: Shellcheck
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - name: Install shellcheck
+        run: sudo apt-get update && sudo apt-get install -y shellcheck
+      - name: Run shellcheck on shell scripts
+        run: |
+          files=$(git ls-files '*.sh' || true)
+          if [ -z "$files" ]; then
+            echo "No shell scripts to check"
+            exit 0
+          fi
+          echo "$files"
+          shellcheck $files
+
+  syntax:
+    name: Syntax & EOL checks
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - name: Fail on CRLF in scripts
+        run: |
+          CRLF_FILES=$(git ls-files '*.sh' | xargs -r grep -Il $'\r' || true)
+          if [ -n "$CRLF_FILES" ]; then
+            echo "CRLF found in the following files:"; echo "$CRLF_FILES";
+            exit 1
+          fi
+          echo "No CRLF in shell scripts"
+      - name: Bash syntax-check
+        run: |
+          for f in $(git ls-files '*.sh'); do
+            echo "Checking $f";
+            bash -n "$f";
+          done
+
+  preview-runner:
+    name: Preview installer scripts
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - name: Bash syntax-check run_installers
+        run: bash -n run_installers.sh
+      - name: Preview run_installers (safe)
+        run: ./run_installers.sh --preview
+
+  windows-preview:
+    name: PowerShell preview
+    runs-on: windows-latest
+    steps:
+      - uses: actions/checkout@v4
+      - name: Preview PowerShell wrapper
+        shell: pwsh
+        run: |
+          ./run_installers.ps1 -Preview