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/embed_stub.go

@@ -0,0 +1,9 @@
+//go:build !withshell
+
+package shellembed
+
+import "embed"
+
+// Untagged builds (tests, vet, plain `go build`) carry no embedded UI;
+// config resolution then requires an explicit shell dir.
+var distFS embed.FS