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

runtime/runtime.go

@@ -1,21 +1,63 @@
 package runtime
 
+import (
+	"path/filepath"
+	"strings"
+)
+
+// Compose labels set by podman-compose on every container it creates.
+const (
+	LabelPodmanComposeProject = "io.podman.compose.project"
+	LabelComposeWorkingDir    = "com.docker.compose.project.working_dir"
+	LabelComposeConfigFiles   = "com.docker.compose.project.config_files"
+)
+
 // Container represents a discovered running container.
 type Container struct {
 	Name    string
 	Image   string
 	Command string
 	Runtime string // "podman" or "docker"
+	User    string // empty = root/system scope
+	Labels  map[string]string
+}
+
+// Project returns the podman-compose project the container belongs to, or "".
+func (c Container) Project() string {
+	return c.Labels[LabelPodmanComposeProject]
+}
+
+// WorkingDir returns the compose project working directory, or "".
+func (c Container) WorkingDir() string {
+	return c.Labels[LabelComposeWorkingDir]
+}
+
+// ComposeFiles returns the compose files used to create the container.
+// Relative paths are relative to WorkingDir.
+func (c Container) ComposeFiles() []string {
+	raw := c.Labels[LabelComposeConfigFiles]
+	if raw == "" {
+		return nil
+	}
+	var files []string
+	for _, f := range strings.Split(raw, ",") {
+		if f = strings.TrimSpace(f); f != "" {
+			files = append(files, filepath.Clean(f))
+		}
+	}
+	return files
 }
 
 // Runtime is the interface all container runtimes must implement.
+// user selects the scope: "" = root/system, otherwise a rootless user.
 type Runtime interface {
 	Name() string
-	ListRunning() ([]Container, error)
-	Exists(name string) (bool, error)
+	ListRunning(user string) ([]Container, error)
+	Exists(user, name string) (bool, error)
 }
 
-// Available returns all supported runtimes (callers skip those with no binary).
+// Available returns all supported runtimes in priority order: podman first,
+// docker as fallback (callers skip those with no binary).
 func Available() []Runtime {
 	return []Runtime{&Podman{}, &Docker{}}
 }