P3.1: Add ExecutionMode/OverlapPolicy to Config; add Pending to JobRuntime

Introduces ExecutionMode (parallel/sequential) and OverlapPolicy
(skip/queue) types and constants in domain/config.go, wires defaults
(parallel/skip) into loadOrCreateConfig and the normalization pass, and
adds validation in validateConfig. Adds Pending bool to JobRuntime as
the flag P3.3 will use to re-run a queued overlap. Marks P3.1 done.

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

src/app/operations_test.go

@@ -25,7 +25,7 @@ func newTempService(t *testing.T, jobs []domain.Job) *Service {
 			JobsPath:       filepath.Join(dir, "jobs.json"),
 			LogsDir:        filepath.Join(dir, "logs"),
 		},
-		Config: domain.Config{JobsDir: ".", LogsDir: "logs", MaxLogFiles: 100, MaxLogAgeDays: 30},
+		Config: domain.Config{JobsDir: ".", LogsDir: "logs", MaxLogFiles: 100, MaxLogAgeDays: 30, ExecutionMode: domain.ExecutionModeParallel, OverlapPolicy: domain.OverlapPolicySkip},
 	}
 	return NewService(store, jobs)
 }