feat: script and exec runtimes with configurable shell

Add runtime: script (shell file, default /bin/bash, optional stop_script) and runtime: exec (executable with args, optional stop_executable). Both support service_type simple|oneshot and working_dir.

README.md

@@ -138,8 +138,43 @@ units:
     compose_bin: /usr/bin/podman-compose
     order: 3
     enabled: true
+
+  - name: backup             # shell script unit
+    runtime: script
+    script: /opt/scripts/backup.sh       # required
+    stop_script: /opt/scripts/stop.sh    # optional → ExecStop
+    shell: /bin/bash                     # optional, default /bin/bash
+    service_type: simple     # simple (default) | oneshot
+    working_dir: /opt/scripts            # optional
+    order: 4
+    enabled: true
+
+  - name: agent              # executable unit
+    runtime: exec
+    executable: /opt/agent/bin/agent     # required
+    args: [--config, /etc/agent.toml]    # optional
+    stop_executable: /opt/agent/bin/stop # optional → ExecStop
+    order: 5
+    enabled: true
 ```
 
+### Script / executable units
+
+Added manually (`unitdore edit`); syncup never creates or disables them.
+
+| Key | Runtime | Meaning |
+|---|---|---|
+| `script` | script | Required. `ExecStart=<shell> <script>` |
+| `stop_script` | script | Optional. `ExecStop=-<shell> <stop_script>` |
+| `shell` | script | Interpreter, default `/bin/bash` |
+| `executable` | exec | Required. `ExecStart=<executable> <args>` |
+| `args` | exec | Optional list, quoted as needed |
+| `stop_executable` | exec | Optional. `ExecStop=-<stop_executable>` |
+| `service_type` | both | `simple` (default) or `oneshot` (RemainAfterExit=yes) |
+| `working_dir` | both | Optional `WorkingDirectory` |
+| `command` | both | If set, overrides ExecStart |
+| `user` | both | Set for a user unit |
+
 ### Stacks (podman-compose)
 
 | Condition | Unit |