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

testdata/stack/compose.yaml

@@ -0,0 +1,15 @@
+# Test stack for unitdore compose-stack detection.
+# Run: podman-compose -p unitdore-test up -d
+name: unitdore-test
+
+services:
+  web:
+    image: docker.io/library/alpine:latest
+    init: true
+    command: ["sleep", "infinity"]
+    depends_on:
+      - db
+  db:
+    image: docker.io/library/alpine:latest
+    init: true
+    command: ["sleep", "infinity"]