docs(gateway): precise watchdog scope, explicit import-resets-activity contract (review S4)

- Config docs now describe session_stall_timeout precisely: a RECOVERY
  notifier for an in-process AIAgent with an adapter-queued follow-up —
  not a general gateway/session stall detector — with a per-AIAgent scan
  cadence (not globally coordinated per durable session).
- import_sessions documents the deliberate export-includes /
  import-resets asymmetry for the activity fields (no resurrected
  'working' labels on machines where no agent runs), with a regression
  pinning both halves.
- Strip trailing whitespace in contributors/emails/fangliquan@qq.com
  (git diff --check housekeeping).

PR #76354 review, scope/contract items + housekeeping.

hermes_state_portability.py

@@ -332,6 +332,15 @@ class SessionPortabilityMixin:
         fail foreign-key validation. Gateway routing, handoff, rewind, and other
         live runtime state are intentionally reset: this restores conversation
         history, not ownership of a live channel or process.
+
+        Activity contract (#76354 review S4): export INCLUDES the live
+        activity fields (``last_activity_at`` / ``last_activity_description``
+        / ``last_activity_provenance``) because they are part of the durable
+        row, but import deliberately RESETS them to NULL. Resurrecting a
+        stale "working ..." label on a machine where no agent is running
+        would fabricate activity the watchdog and session listings act on.
+        This asymmetry is intentional and covered by regression
+        (tests/gateway/test_watchdog_review_76354.py::test_s4_export_includes_activity_import_resets_it).
         """
         if not isinstance(sessions, list):
             raise ValueError("sessions must be a list")