Rename project to GoSentry

Rename the application, Go module path, command package, build artifacts, resource script, and embedded icon assets from PySentry/pysentry to GoSentry/gosentry.

Move portable settings to gosentry.yaml while reading legacy pysentry.yaml during the transition, then rewrite settings under the new name.

Update Windows and Linux autostart integration to use GoSentry names while cleaning up legacy PySentry registry, desktop-entry, and systemd artifacts.

Refresh README, architecture notes, roadmap, changelog, and release examples for version 0.3.0.

src/core/scheduler.go

@@ -222,7 +222,7 @@ func nextRunTime(schedule string, from time.Time) (time.Time, bool) {
 		}
 		return from.Add(interval), true
 	}
-	// Standard five-field cron keeps PySentry compatible with the mental model
+	// Standard five-field cron keeps GoSentry compatible with the mental model
 	// users already know from Unix cron, while robfig/cron handles edge cases
 	// such as ranges, steps, and day-of-week names.
 	parsed, err := cronParser.Parse(schedule)