T1.1: Create src/domain; move Job/RunRecord/Config/JobsFile/StartInTrayArgument

Extracts the five domain types out of src/core/model.go into a new
src/domain package (job.go, record.go, config.go). The unexported
nextDue field is promoted to NextDue so it is accessible from core.
All references across src/core, src/gui, and cmd/gosentry are updated
to use domain.TypeName. src/core/model.go is reduced to a bare package
declaration. Windows and Linux cross-compilation both pass; all tests
remain green.

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

cmd/gosentry/main.go

@@ -3,7 +3,7 @@ package main
 import (
 	"os"
 
-	"gitea.mixdep.ru/mix/gosentry/src/core"
+	"gitea.mixdep.ru/mix/gosentry/src/domain"
 	"gitea.mixdep.ru/mix/gosentry/src/gui"
 )
 
@@ -11,7 +11,7 @@ func main() {
 	// The executable entry point intentionally delegates all startup work to the
 	// GUI package. Keeping main small makes it easier to add platform-specific
 	// packaging later without mixing window setup, storage, and scheduler logic.
-	gui.Run(hasArgument(core.StartInTrayArgument))
+	gui.Run(hasArgument(domain.StartInTrayArgument))
 }
 
 func hasArgument(argument string) bool {