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

docs/generated-units.md

@@ -277,6 +277,59 @@ units:
 
 ---
 
+## Stack Unit (podman-compose, compose mode)
+
+Used when `compose_bin`, `working_dir` and all `compose_files` exist.
+
+```yaml
+- name: shop
+  kind: stack
+  runtime: podman
+  project: shop
+  working_dir: /srv/shop
+  compose_files: [compose.yaml]
+  compose_bin: /usr/bin/podman-compose
+  enabled: true
+```
+
+```ini
+# /etc/systemd/system/unitdore-shop.service
+# Generated by unitdore — do not edit manually
+
+[Unit]
+Description=Unitdore: shop (podman-compose stack)
+After=network.target
+
+[Service]
+Type=simple
+WorkingDirectory=/srv/shop
+ExecStart=/usr/bin/podman-compose -p shop -f compose.yaml up
+ExecStop=-/usr/bin/podman-compose -p shop -f compose.yaml stop
+
+[Install]
+WantedBy=multi-user.target
+```
+
+---
+
+## Stack Unit (pod fallback)
+
+Used when the compose binary, working dir or a compose file is missing. Requires pod `pod_<project>`.
+
+```ini
+[Unit]
+Description=Unitdore: shop (podman pod)
+After=network.target
+
+[Service]
+Type=oneshot
+RemainAfterExit=yes
+ExecStart=/usr/bin/podman pod start pod_shop
+ExecStop=-/usr/bin/podman pod stop pod_shop
+```
+
+---
+
 ## Quick Reference: Service File Locations
 
 | Scenario | Path |