feat: podman-compose stack units and root/user execution scopes

- syncup: podman first, docker only as fallback (podman wins on name clash;
  existing docker units found in podman switch to podman)
- syncup: group podman-compose containers into one `kind: stack` unit per
  project (project, working_dir, compose_files, compose_bin from labels);
  disable existing member units ("part of stack <project>")
- generator: compose mode (podman-compose up/stop, WorkingDirectory) with
  pod start/stop fallback when compose binary/dir/files are missing
- reconcile: stacks disabled with "stack not found" when neither compose
  files nor pod exist
- root: scan root + rootless users listed in `users:` (podman only)
- non-root: config ~/.config/unitdore/units.yaml, own user units only
- sysuser package: runuser with XDG_RUNTIME_DIR/DBUS env, home/linger lookup
- install: warn when linger is not enabled for user units
- testdata/stack: podman-compose test stack and captured podman ps fixture
- docs: README, generated-units, man page, AGENTS, PLAN

config/config_test.go

@@ -144,3 +144,42 @@ func TestDisabledReason_Roundtrip(t *testing.T) {
 		t.Errorf("expected disabled_reason to survive roundtrip, got: %q", loaded.Units[0].DisabledReason)
 	}
 }
+
+func TestSaveAndLoad_Stack(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "units.yaml")
+	cfg := &Config{
+		Users: []string{"hein"},
+		Units: []Unit{{
+			Name: "shop", Runtime: "podman", Kind: KindStack, Project: "shop",
+			WorkingDir: "/srv/shop", ComposeFiles: []string{"compose.yaml"},
+			ComposeBin: "/usr/bin/podman-compose", User: "hein", Enabled: true,
+		}},
+	}
+	if err := cfg.Save(path); err != nil {
+		t.Fatalf("Save failed: %v", err)
+	}
+	loaded, err := Load(path)
+	if err != nil {
+		t.Fatalf("Load failed: %v", err)
+	}
+	u := loaded.Units[0]
+	if !u.IsStack() || u.Project != "shop" || u.WorkingDir != "/srv/shop" ||
+		len(u.ComposeFiles) != 1 || u.ComposeBin != "/usr/bin/podman-compose" {
+		t.Errorf("stack fields not round-tripped: %+v", u)
+	}
+	if len(loaded.Users) != 1 || loaded.Users[0] != "hein" {
+		t.Errorf("users not round-tripped: %v", loaded.Users)
+	}
+}
+
+func TestIsStack_DefaultKind(t *testing.T) {
+	if (Unit{Name: "x"}).IsStack() {
+		t.Error("unit without kind must be a container")
+	}
+}
+
+func TestUserConfigPath(t *testing.T) {
+	if got := UserConfigPath("/home/hein"); got != "/home/hein/.config/unitdore/units.yaml" {
+		t.Errorf("UserConfigPath() = %q", got)
+	}
+}