feat: add nix for formatting

Also adds CI tests!

.forgejo/workflows/check.yml

@@ -0,0 +1,41 @@
+on:
+  push:
+jobs:
+  check:
+    runs-on: nixos
+    strategy:
+      matrix:
+        check:
+          - treefmt
+    steps:
+      - uses: "https://git.salame.cl/actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683" # v4
+      - name: Run checks
+        run: |
+          nix --version
+          # shellcheck disable=SC2016
+          nix build --print-build-logs '.#checks.x86_64-linux.${{ matrix.check }}'
+  check-renovaterc:
+    runs-on: nixos
+    steps:
+      - uses: "https://git.salame.cl/actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683" # v4
+      - name: Validate renovaterc.json
+        run: |
+          nix --version
+          nix shell nixpkgs#renovate --command renovate-config-validator
+  report-size:
+    runs-on: nixos
+    needs: check
+    steps:
+      - uses: "https://git.salame.cl/actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683" # v4
+      - run: nix --version
+      - name: Create Size Report
+        uses: ./
+        with:
+          # Create a comment on the associated PR
+          comment-on-pr: ${{ github.ref_name != 'main' }}
+          # Generate artifacts on main (to speed up comparisons)
+          generate-artifact: ${{ github.ref_name == 'main' }}
+          # Generate comparisons to main
+          do-comparison: 'true'
+          # This job's name (so we can find the artifacts)
+          job-name: report-size