nix: add dependabot and flake update automation (#24)

.github/dependabot.yml

@@ -0,0 +1,9 @@
+version: 2
+updates:
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    target-branch: "main"
+    schedule:
+      interval: "weekly"
+    commit-message:
+      prefix: "ci:"

.github/workflows/update_flake.yml

@@ -0,0 +1,20 @@
+name: Update flake inputs
+on:
+  workflow_dispatch: # allows manual triggering
+  schedule:
+    - cron: '0 0 * * 0' # runs weekly on Sunday at 00:00
+
+jobs:
+  lockfile:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+      - name: Install Nix
+        uses: DeterminateSystems/nix-installer-action@main
+      - name: Update flake.lock
+        uses: DeterminateSystems/update-flake-lock@main
+        with:
+          pr-title: "Update flake.lock"
+          pr-labels: |
+            dependencies