fix: resolve golangci-lint failures for new lint rules

golangci-lint v2.x (via golangci-lint-action@v9) enforces errcheck
more strictly. Fix unchecked os.Setenv/os.Unsetenv calls in example
tests and unchecked fmt.Fprintf return values in usage.go.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: mtingers

example_test.go

@@ -8,10 +8,14 @@ import (
 )
 
 func ExampleProcess() {
-	os.Setenv("APP_HOST", "localhost")
-	os.Setenv("APP_PORT", "8080")
-	defer os.Unsetenv("APP_HOST")
-	defer os.Unsetenv("APP_PORT")
+	if err := os.Setenv("APP_HOST", "localhost"); err != nil {
+		panic(err)
+	}
+	if err := os.Setenv("APP_PORT", "8080"); err != nil {
+		panic(err)
+	}
+	defer func() { _ = os.Unsetenv("APP_HOST") }()
+	defer func() { _ = os.Unsetenv("APP_PORT") }()
 
 	type Config struct {
 		Host string
@@ -28,10 +32,14 @@ func ExampleProcess() {
 }
 
 func ExampleProcess_nested() {
-	os.Setenv("APP_DATABASE_HOST", "db.local")
-	os.Setenv("APP_DATABASE_PORT", "5432")
-	defer os.Unsetenv("APP_DATABASE_HOST")
-	defer os.Unsetenv("APP_DATABASE_PORT")
+	if err := os.Setenv("APP_DATABASE_HOST", "db.local"); err != nil {
+		panic(err)
+	}
+	if err := os.Setenv("APP_DATABASE_PORT", "5432"); err != nil {
+		panic(err)
+	}
+	defer func() { _ = os.Unsetenv("APP_DATABASE_HOST") }()
+	defer func() { _ = os.Unsetenv("APP_DATABASE_PORT") }()
 
 	type DB struct {
 		Host string

usage.go

@@ -64,7 +64,7 @@ func usageStruct(prefix string, rt reflect.Type, tw *tabwriter.Writer) {
 			opts = fmt.Sprintf("[default: %s]", spec.DefaultValue)
 		}
 
-		fmt.Fprintf(tw, "  %s\t%s\t%s\t%s\n", key, typeName, opts, spec.Description)
+		_, _ = fmt.Fprintf(tw, "  %s\t%s\t%s\t%s\n", key, typeName, opts, spec.Description)
 	}
 }
 
@@ -77,7 +77,7 @@ func writeUsage(prefix string, spec interface{}, out io.Writer) error {
 		return fmt.Errorf("envstruct: spec must be a struct or pointer to struct")
 	}
 	tw := tabwriter.NewWriter(out, 0, 4, 2, ' ', 0)
-	fmt.Fprintf(tw, "  %s\t%s\t%s\t%s\n", "KEY", "TYPE", "DEFAULT", "DESCRIPTION")
+	_, _ = fmt.Fprintf(tw, "  %s\t%s\t%s\t%s\n", "KEY", "TYPE", "DEFAULT", "DESCRIPTION")
 	usageStruct(prefix, rv.Type(), tw)
 	return tw.Flush()
 }