Added a convention that sentinel errors go near the top of the file, and applied it

internal/config/section_test.go

@@ -10,6 +10,9 @@ import (
 	"git.avsdev.uk/Go/job-search/internal/util"
 )
 
+// errRejected is the error testValidated.validate rejects "bad" with.
+var errRejected = errors.New("rejected")
+
 // Test sections exercising every field shape section.go supports.
 type testInner struct {
 	Name string `cfg:"name" default:"inner"`
@@ -51,8 +54,6 @@ type testValidated struct {
 	Value string `cfg:"value"`
 }
 
-var errRejected = errors.New("rejected")
-
 func (c *testValidated) validate(key string, value any) error {
 	if key == "value" && value.(string) == "bad" {
 		return errRejected