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

cmd/helpers.go

@@ -0,0 +1,25 @@
+package cmd
+
+import (
+	"fmt"
+
+	"github.com/warkanum/unitdore/config"
+	"github.com/warkanum/unitdore/sysuser"
+)
+
+// runtimeLabel is the RUNTIME column value for list/status.
+func runtimeLabel(u config.Unit) string {
+	if u.IsStack() {
+		return u.Runtime + " (stack)"
+	}
+	return u.Runtime
+}
+
+// lingerHint prints, once per user, how to make user units start at boot.
+func lingerHint(seen map[string]bool, u config.Unit) {
+	if u.User == "" || seen[u.User] || sysuser.LingerEnabled(u.User) {
+		return
+	}
+	seen[u.User] = true
+	fmt.Printf("  ! linger not enabled for %s — user units won't start at boot: sudo loginctl enable-linger %s\n", u.User, u.User)
+}