tools/bump: reformat after the toolchain moves

nixpkgs decides which prettier and gofumpt format the tree. A lock bump to
prettier 3.9.6 reformatted CHANGELOG.md, which no area had touched, and the
formatting check failed on a file the bot never edited.

tools/bump/area.go

@@ -82,13 +82,23 @@ func allAreas() []area {
 	areas = append(areas, imageAreas()...)
 	areas = append(areas, actionAreas()...)
 
-	return append(areas, area{
+	areas = append(areas, area{
 		Name:    "generate",
 		Needs:   []string{"gomod", "tools:oapi-codegen"},
 		Apply:   applyGenerate,
 		Gate:    gateGenerate,
 		Message: func(change) string { return "all: regenerate generated files" },
 	})
+
+	// Last, so it also covers whatever the generators emitted. nixpkgs decides
+	// which prettier and gofumpt the tree is formatted with, so a lock bump can
+	// reformat files no other area went near.
+	return append(areas, area{
+		Name:    "fmt",
+		Needs:   []string{"flake"},
+		Apply:   applyFormat,
+		Message: func(change) string { return "all: apply the formatters from the new toolchain" },
+	})
 }
 
 // runAreas applies each area in order, committing the ones that hold and

tools/bump/generate.go

@@ -129,3 +129,27 @@ func gateGenerate(ctx context.Context, r *repo) error {
 
 	return nil
 }
+
+// applyFormat re-runs the repository formatters. A toolchain bump can change
+// what "formatted" means: a newer prettier out of nixpkgs reformats files no
+// bump touched, and the formatting check then fails on a tree the bot never
+// edited. Running last means it also tidies whatever the generators emitted.
+func applyFormat(ctx context.Context, r *repo) (change, error) {
+	if _, err := r.nixRun(ctx, "make", "fmt"); err != nil { //nolint:noinlineerr
+		return change{}, err
+	}
+
+	touched, err := changedFiles(ctx, r)
+	if err != nil {
+		return change{}, err
+	}
+
+	if len(touched) == 0 {
+		return change{Empty: true}, nil
+	}
+
+	return change{
+		Summary: fmt.Sprintf("%d file(s) reformatted", len(touched)),
+		Detail:  touched,
+	}, nil
+}