chore: round-2 review nits (re-review #9)

- tests/agent/test_session_activity.py asserts against
  ACTIVITY_DESCRIPTION_MAX instead of the literal 120.
- The session-stall WARNING log line names its config knob
  (agent.session_stall_timeout) so operators can find the setting.
- hermes_state.py: collapse the triple blank line near line 191.
- hermes_cli/status.py no longer imports the private
  hermes_cli.main._relative_time: the helper moved to a public home
  (hermes_cli.timefmt.relative_time); main._relative_time stays as a
  thin back-compat wrapper (sessions_cmd and external patchers keep
  working).

tests/agent/test_session_activity.py

@@ -3,6 +3,7 @@
 from types import SimpleNamespace
 
 from agent.session_activity import (
+    ACTIVITY_DESCRIPTION_MAX,
     ActivityProvenance,
     bound_activity_description,
     build_activity_snapshot,
@@ -12,9 +13,9 @@ from agent.session_activity import (
 
 
 def test_bound_activity_description_truncates():
-    long = "x" * 200
+    long = "x" * (ACTIVITY_DESCRIPTION_MAX + 80)
     out = bound_activity_description(long)
-    assert len(out) == 120
+    assert len(out) == ACTIVITY_DESCRIPTION_MAX
     assert out.endswith("…")