ci: add CI workflow

Gives the github-actions dependabot ecosystem workflow files to track
and adds basic checks on push and pull request.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: fey

.github/workflows/ci.yml

@@ -0,0 +1,18 @@
+name: CI
+
+on:
+  - push
+  - pull_request
+
+jobs:
+  validate:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v6
+      - uses: hashicorp/setup-terraform@v3
+      - name: Check formatting
+        run: terraform fmt -check
+      - name: Init
+        run: terraform init -backend=false
+      - name: Validate
+        run: terraform validate