Add remain:yes option for run/task oneshot commands

Similar to systemd's RemainAfterExit=yes.  Prevents the task from
re-running on runlevel re-entry and ensures the post: script runs
when explicitly stopped or when leaving valid runlevels.

Useful for tasks that set up persistent state like firewall rules:

    task [2345] remain:yes \
         post:/usr/sbin/teardown-firewall \
         /usr/sbin/setup-firewall -- Firewall setup

Not supported for bootstrap-only tasks (runlevel S only) since these
are deleted immediately after completion.

doc/config/service-opts.md

@@ -69,6 +69,11 @@ confines of that the following options are available:
   * `respawn` -- bypasses the `restart` mechanism completely, allows
     endless restarts.  Useful in many use-cases, but not what `service`
     was originally designed for so not the default behavior
+  * `remain:yes` -- for `run` and `task` only.  Prevents the task from
+    re-running on runlevel re-entry and ensures the `post:` script runs
+    when the task is explicitly stopped or leaves its valid runlevels.
+    Similar to systemd's `RemainAfterExit=yes`.  See [Task and Run](task-and-run.md)
+    for more details
   * `oncrash:reboot` -- when all retries have failed, and the service
     has *crashed*, if this option is set the system is rebooted
   * `oncrash:script` -- similarly, but instead of rebooting, call the

doc/config/task-and-run.md

@@ -44,3 +44,44 @@ redirects can be used:
 
 Please note, `;`, `&&`, `||`, and similar are *not supported*.  Any
 non-trivial constructs are better placed in a separate shell script.
+
+
+remain:yes
+----------
+
+By default, a `run` or `task` will re-run each time its runlevel is
+entered, and its `post:` script does not run on completion.
+
+With `remain:yes`, the task runs once and does not re-run on runlevel
+re-entry:
+
+    task [2345] remain:yes /usr/sbin/setup-firewall -- Firewall setup
+
+This has the following effects:
+
+  * The task does not re-run on runlevel re-entry
+  * The `post:` script runs when:
+    - The task is explicitly stopped (`initctl stop NAME`)
+    - The task leaves its valid runlevels (e.g., runlevel change)
+
+This is useful for tasks that set up persistent state where:
+
+  * Cleanup should only happen on explicit stop or when leaving valid runlevels
+  * The setup should not be re-run on every runlevel entry
+
+**Example:** Setting up firewall rules with cleanup on shutdown:
+
+```
+task [2345] remain:yes \
+     post:/usr/sbin/teardown-firewall \
+     /usr/sbin/setup-firewall -- Firewall setup
+```
+
+The firewall rules are created once.  The `post:` script runs when
+entering runlevel 0 (halt) or 6 (reboot), or on explicit stop.
+
+> [!NOTE]
+> The `remain:yes` option is not supported for bootstrap-only tasks
+> (tasks with only runlevel S).  Bootstrap tasks are deleted immediately
+> after completion, and their `post:` scripts never run.  A warning is
+> logged if `remain:yes` is used on such tasks.