feat(01-03): add summer dev watch rebuild loop

- Watch app sources with fsnotify, debounce, and one serialized build.App
- Restart the child only after a successful build and print rebuild latency
- Ignore generated app files and reap the child on cancellation

cmd/summer/main_test.go

@@ -19,7 +19,7 @@ func TestToolCommandNames(t *testing.T) {
 	for _, c := range toolCommands() {
 		names = append(names, c.Name)
 	}
-	for _, want := range []string{"build", "make:plugin", "plugin:add"} {
+	for _, want := range []string{"build", "make:plugin", "plugin:add", "dev"} {
 		if !slices.Contains(names, want) {
 			t.Fatalf("missing %s in %v", want, names)
 		}