Add Forgejo runner and check update action

.forgejo/workflows/auto-update.yaml

@@ -0,0 +1,72 @@
+name: Weekly Flake Update
+
+on:
+  schedule:
+    # Every Friday at 22:00 UTC
+    - cron: '0 22 * * 5'
+  workflow_dispatch: {}
+
+jobs:
+  update:
+    runs-on: nix
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+
+      - name: Configure Git
+        run: |
+          git config user.name "Flake Update Bot"
+          git config user.email "bot@noreply.local"
+
+      - name: Update flake inputs
+        run: nix flake update
+
+      - name: Check for changes
+        id: changes
+        run: |
+          if git diff --quiet flake.lock; then
+            echo "changed=false" >> $GITHUB_OUTPUT
+          else
+            echo "changed=true" >> $GITHUB_OUTPUT
+          fi
+
+      - name: Build all hosts
+        if: steps.changes.outputs.changed == 'true'
+        run: |
+          nix develop -c colmena build
+
+      - name: Create branch and commit
+        if: steps.changes.outputs.changed == 'true'
+        run: |
+          BRANCH_NAME="auto-update/$(date +%Y-%m-%d)"
+          git checkout -b "$BRANCH_NAME"
+          git add flake.lock
+          git commit -m "chore: update flake inputs $(date +%Y-%m-%d)"
+          git push origin "$BRANCH_NAME"
+          echo "branch_name=$BRANCH_NAME" >> $GITHUB_OUTPUT
+        id: branch
+
+      - name: Create Pull Request
+        if: steps.changes.outputs.changed == 'true'
+        env:
+          FORGEJO_TOKEN: ${{ secrets.FORGEJO_TOKEN }}
+        run: |
+          curl -X POST \
+            -H "Authorization: token $FORGEJO_TOKEN" \
+            -H "Content-Type: application/json" \
+            -d '{
+              "title": "chore: weekly flake update",
+              "body": "Automated flake update from CI.\n\nThis PR updates all flake inputs and has been verified to build successfully.\n\nGenerated on: '"$(date -Iseconds)"'",
+              "head": "'"${{ steps.branch.outputs.branch_name }}"'",
+              "base": "master"
+            }' \
+            "${{ github.server_url }}/api/v1/repos/${{ github.repository }}/pulls"
+
+      - name: Cleanup
+        if: always()
+        run: |
+          nix-collect-garbage -d
+          rm -rf result .direnv
+