feat(ci): Add Forgejo Actions CI workflow

Set up continuous integration for nixos-configs:
- Validate flake syntax with `nix flake check --no-build`
- Display flake metadata and outputs for visibility
- Build critical NixOS configurations in parallel matrix
- Configurations tested: john-endesktop, zix790prors, nix-book, boxy

Implements bead: nixos-configs-2hq

.forgejo/workflows/ci.yaml

@@ -0,0 +1,47 @@
+name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+jobs:
+  flake-check:
+    runs-on: nix
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Check flake syntax
+        run: nix flake check --no-build
+
+      - name: Show flake metadata
+        run: nix flake metadata
+
+      - name: Show flake outputs
+        run: nix flake show
+
+  build-configs:
+    runs-on: nix
+    needs: flake-check
+    strategy:
+      fail-fast: false
+      matrix:
+        config:
+          # Critical server configurations
+          - john-endesktop
+          # Desktop configurations
+          - zix790prors
+          - nix-book
+          # Media center
+          - boxy
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Build ${{ matrix.config }}
+        run: |
+          nix build .#nixosConfigurations.${{ matrix.config }}.config.system.build.toplevel \
+            --no-link \
+            --print-build-logs