fix(process-registry): use portable /bin/sh probe for systemd-run scope availability (#105365)

tools/process_registry.py

@@ -207,7 +207,12 @@ def _systemd_run_user_scope_available() -> bool:
     """True if ``systemd-run --user --scope`` can create a cgroup.
     ``shutil.which`` alone is insufficient: system services and containers may lack
     the user D-Bus bus even with the binary on PATH (every spawn would fail with
-    ``Failed to connect to user bus``), so a cheap ``/bin/true`` probe is run and cached."""
+    ``Failed to connect to user bus``), so a cheap probe is run and cached.
+
+    The probe payload is ``/bin/sh -c 'exit 0'`` rather than ``/bin/true``: NixOS has
+    no FHS ``/bin`` (only ``sh``), so an absolute ``/bin/true`` probe fails there for a
+    reason unrelated to scope availability and disables restart-safe cron dispatch on
+    every scheduled fire (#105365).``"""
     global _SYSTEMD_SCOPE_AVAILABLE, _SYSTEMD_SCOPE_PROBED_AT
     verdict = _systemd_scope_cached()
     if verdict is not None:
@@ -228,7 +233,7 @@ def _systemd_run_user_scope_available() -> bool:
                     # Unique unit avoids collisions; the timeout bounds D-Bus.
                     probe_unit = f"hermes-probe-scope-{os.getpid()}-{uuid.uuid4().hex[:8]}"
                     result = subprocess.run(
-                        _systemd_scope_argv(binary, probe_unit, "/bin/true"),
+                        _systemd_scope_argv(binary, probe_unit, "/bin/sh", "-c", "exit 0"),
                         capture_output=True,
                         timeout=3,
                         env=systemd_user_bus_env(),