P1.6: update tests and docs for JSON storage + exit-code removal

- storage/store_test.go: switch TestJobsYAMLDoesNotPersistRuntimeNoise to
  json.Marshal (renamed to TestJobsJSONDoesNotPersistRuntimeNoise); add
  TestLoadOrCreateJobsMigratesFromLegacy to cover the jobs YAML import path
- app/operations_test.go: update newTempService paths to .json filenames
- docs/TESTS.md: reflect JSON round-trip, drop SuccessExitCodes tests, remove
  Exit Code Flexibility design-principle entry
- docs/PRE-RELEASE-TASKS.md: mark P1.6 complete

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

src/app/operations_test.go

@@ -20,9 +20,9 @@ func newTempService(t *testing.T, jobs []domain.Job) *Service {
 		Paths: storage.Paths{
 			ExecutablePath: filepath.Join(dir, "gosentry"),
 			AppDir:         dir,
-			ConfigPath:     filepath.Join(dir, "gosentry.yaml"),
+			ConfigPath:     filepath.Join(dir, "gosentry.json"),
 			JobsDir:        dir,
-			JobsPath:       filepath.Join(dir, "jobs.yaml"),
+			JobsPath:       filepath.Join(dir, "jobs.json"),
 			LogsDir:        filepath.Join(dir, "logs"),
 		},
 		Config: domain.Config{JobsDir: ".", LogsDir: "logs", MaxLogFiles: 100, MaxLogAgeDays: 30},