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"]

testdata/stack/podman_ps.json

@@ -0,0 +1,100 @@
+[
+  {
+    "AutoRemove": false,
+    "Command": [
+      "sleep",
+      "infinity"
+    ],
+    "CreatedAt": "6 seconds ago",
+    "CIDFile": "",
+    "Exited": false,
+    "ExitedAt": -62135596800,
+    "ExitCode": 0,
+    "ExposedPorts": null,
+    "Id": "6d5a1e4d8ab39d64b26842a0e50b5943cfa62d71c70d63ce5c2ad92f598811b9",
+    "Image": "docker.io/library/alpine:latest",
+    "ImageID": "a40c03cbb81c59bfb0e0887ab0b1859727075da7b9cc576a1cec2c771f38c5fb",
+    "IsInfra": false,
+    "Labels": {
+      "PODMAN_SYSTEMD_UNIT": "podman-compose@unitdore-test.service",
+      "com.docker.compose.container-number": "1",
+      "com.docker.compose.project": "unitdore-test",
+      "com.docker.compose.project.config_files": "compose.yaml",
+      "com.docker.compose.project.working_dir": "/home/warkanum/dev/unitdore/testdata/stack",
+      "com.docker.compose.service": "db",
+      "io.podman.compose.config-hash": "1a54723a4ead4531f2ddce5aa7e85019b4ebd1612ca8973b85cd9026f4f87949",
+      "io.podman.compose.project": "unitdore-test",
+      "io.podman.compose.service": "db",
+      "io.podman.compose.version": "1.6.0"
+    },
+    "Mounts": [],
+    "Names": [
+      "unitdore-test_db_1"
+    ],
+    "Namespaces": {
+      
+    },
+    "Networks": [
+      "unitdore-test_default"
+    ],
+    "Pid": 75799,
+    "Pod": "235775e179ed8413b84b823ff63a812a053d929b8399b2d48cc19f9a4eeef410",
+    "PodName": "",
+    "Ports": null,
+    "Restarts": 0,
+    "Size": null,
+    "StartedAt": 1791823934,
+    "State": "running",
+    "Status": "Up 6 seconds",
+    "Created": 1791823933
+  },
+  {
+    "AutoRemove": false,
+    "Command": [
+      "sleep",
+      "infinity"
+    ],
+    "CreatedAt": "6 seconds ago",
+    "CIDFile": "",
+    "Exited": false,
+    "ExitedAt": -62135596800,
+    "ExitCode": 0,
+    "ExposedPorts": null,
+    "Id": "b6b364f2a885baa583b7112cd7d37832e7f11f9091698ef76ca4c44259e501cc",
+    "Image": "docker.io/library/alpine:latest",
+    "ImageID": "a40c03cbb81c59bfb0e0887ab0b1859727075da7b9cc576a1cec2c771f38c5fb",
+    "IsInfra": false,
+    "Labels": {
+      "PODMAN_SYSTEMD_UNIT": "podman-compose@unitdore-test.service",
+      "com.docker.compose.container-number": "1",
+      "com.docker.compose.project": "unitdore-test",
+      "com.docker.compose.project.config_files": "compose.yaml",
+      "com.docker.compose.project.working_dir": "/home/warkanum/dev/unitdore/testdata/stack",
+      "com.docker.compose.service": "web",
+      "io.podman.compose.config-hash": "f38934df837f52ead7f2848b58acef10380a7929125a7c4f61199ad5b557302f",
+      "io.podman.compose.project": "unitdore-test",
+      "io.podman.compose.service": "web",
+      "io.podman.compose.version": "1.6.0"
+    },
+    "Mounts": [],
+    "Names": [
+      "unitdore-test_web_1"
+    ],
+    "Namespaces": {
+      
+    },
+    "Networks": [
+      "unitdore-test_default"
+    ],
+    "Pid": 75919,
+    "Pod": "235775e179ed8413b84b823ff63a812a053d929b8399b2d48cc19f9a4eeef410",
+    "PodName": "",
+    "Ports": null,
+    "Restarts": 0,
+    "Size": null,
+    "StartedAt": 1791823934,
+    "State": "running",
+    "Status": "Up 6 seconds",
+    "Created": 1791823933
+  }
+]