feat: Split CI/CD into three focused workflows

Replaced the monolithic pipeline with three dedicated workflows: ci.yml
(build on every push), cd.yml (manual deploy to staging or prod), and
nightly.yml (scheduled build and deploy). This makes it easier to trigger
deploys independently of builds and to diagnose failures.

.forgejo/workflows/ci.yml

@@ -0,0 +1,19 @@
+name: CI
+
+on:
+  push:
+  pull_request:
+
+jobs:
+  build:
+    runs-on: self-hosted
+    steps:
+      - uses: actions/checkout@v4
+      - name: Build
+        run: nix shell nixpkgs#mdbook --command ./deploy.sh build
+      - name: Upload artifact
+        uses: actions/upload-artifact@v3
+        with:
+          name: site-${{ github.sha }}
+          path: blog/
+