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/domain/runtime.go

@@ -18,6 +18,10 @@ type JobRuntime struct {
 	// scheduler comparisons. NextRun above is its formatted display string and is
 	// the only form shown in the GUI.
 	NextDue time.Time
+
+	// Pending is set when a run was skipped due to the overlap policy being
+	// "queue". The scheduler will start this job as soon as the current run ends.
+	Pending bool
 }
 
 // NewRuntime builds the initial runtime state for a freshly loaded or created