ci: fail on code that is not gofmt'd

go vet says nothing about import order, so when the move to git.ryuvia.com
rewrote every import path without re-sorting them -- the new path sorts before
github.com/..., where the old one sorted after -- both repos went through a
green CI run and a release unformatted.

Added to the release workflow as well as CI, so the two keep running the same
checks; ci.yaml's header claims exactly that, and a check in one but not the
other would quietly make it false.

The step handles gofmt's two failure modes separately because they do not look
alike: a misformatted file is listed on stdout with exit 0, so the failure has
to be raised by hand, while a file that does not parse prints nothing to stdout
and exits 2 -- which a plain emptiness test reads as success. Verified against
all three cases (clean, misformatted, unparseable) before committing.

.gitea/workflows/ci.yaml

@@ -59,6 +59,30 @@ jobs:
             git clone --depth=1 --branch "$REF_NAME" "$REPO_URL" .
           fi
 
+      # This exists because `go vet` does not look at import order: the move to
+      # git.ryuvia.com rewrote every import path without re-sorting, the new path sorts
+      # before github.com/..., and both repos sat unformatted through a green CI run and
+      # a release before anyone noticed.
+      #
+      # Both of gofmt's failure modes need handling, and they are not alike. A file that
+      # is merely misformatted is listed on stdout with exit 0 -- so the failure has to
+      # be raised by hand. A file that does not parse is the opposite: nothing on stdout
+      # and exit 2, which a naive `[ -n "$unformatted" ]` reads as success. The first
+      # draft of this step had exactly that hole.
+      - name: Format
+        run: |
+          if ! unformatted=$(gofmt -l .); then
+            echo "::error::gofmt could not parse the tree"
+            gofmt -l .    # re-run unredirected so the parse errors reach the log
+            exit 1
+          fi
+          if [ -n "$unformatted" ]; then
+            echo "::error::not gofmt'd:"
+            echo "$unformatted"
+            gofmt -d .
+            exit 1
+          fi
+
       - name: Vet
         run: go vet ./...