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/install.go

@@ -35,6 +35,7 @@ func runInstall(cmd *cobra.Command, args []string) error {
 	installed := 0
 	skipped := 0
 	removed := 0
+	lingerSeen := map[string]bool{}
 
 	for _, u := range cfg.Units {
 		if !u.Enabled {
@@ -73,6 +74,7 @@ func runInstall(cmd *cobra.Command, args []string) error {
 		} else {
 			path, _ := systemd.UnitPath(u, prefix, suffix)
 			fmt.Printf("  ✓ installed: %s\n", path)
+			lingerHint(lingerSeen, u)
 			installed++
 		}
 	}