core: migrate to dankgo shared go modules, embed quickshell in DMS

binary to mount at runtime, -c or DMS_SHELL_DIR overrides required to
explicitly override embedded configuration

core/internal/shellembed/shellembed_test.go

@@ -0,0 +1,9 @@
+package shellembed
+
+import "testing"
+
+func TestAvailableFalseWithoutTag(t *testing.T) {
+	if Available() {
+		t.Fatal("Available() should be false in untagged test builds")
+	}
+}