refactor(gateway): simplify auto-resume + extend to crash recovery

Follow-up on top of @kyan12's PR #20888 — same feature, cleaner shape,
wider coverage.

Changes:
- Drop the synthetic '[System note: ...]' in the internal MessageEvent.
  The existing _is_resume_pending branch in _handle_message_with_agent
  (run.py ~L13738) already injects a reason-aware recovery system note
  on the next turn.  With kyan's text in place the model saw two stacked
  system notes.  Now the event text is empty and the existing injection
  path owns the wording.
- Drop SessionStore.list_resume_pending() as a new public method.  The
  filter is 8 lines inline in _schedule_resume_pending_sessions() —
  one caller, no other pluggability need.
- Add 'restart_interrupted' to the auto-resume reason set.  That's the
  reason SessionStore.suspend_recently_active() stamps on sessions
  recovered from a crash/OOM/SIGKILL (no .clean_shutdown marker).
  Previously those sessions had to wait for a real user message to
  auto-resume; now they continue automatically at startup like
  drain-timeout interruptions do.
- Reasons live in a _AUTO_RESUME_REASONS frozenset at class scope so
  future reasons (e.g. 'manual_resume_request') can be opted in with
  one line.

Test coverage added:
- drain-timeout + crash-recovery both scheduled
- stale entries skipped (outside freshness window)
- suspended entries skipped (suspended > resume_pending)
- originless entries skipped (no routing target)
- disallowed reasons skipped (graceful forward-compat)

E2E verified end-to-end with a real on-disk SessionStore: 2 eligible
sessions scheduled, 2 ineligible skipped, empty-text internal events
delivered to the adapter.

Co-authored-by: Kevin Yan <kevyan1998@gmail.com>

gateway/session.py

@@ -1028,42 +1028,6 @@ class SessionStore:
             self._save()
             return True
 
-    def list_resume_pending(
-        self,
-        *,
-        window_secs: Optional[float] = None,
-        now: Optional[float] = None,
-        allowed_reasons: Optional[set[str]] = None,
-    ) -> List[SessionEntry]:
-        """Return fresh restart-interrupted sessions eligible for resume.
-
-        Only entries that still have an origin are returned; the gateway needs
-        that origin to route continuation back through the original
-        platform/chat/thread.  ``suspended`` entries are excluded because
-        explicit suspension/stuck-loop escalation must win over resume.
-        """
-        current = datetime.fromtimestamp(now) if now is not None else _now()
-        window = float(window_secs) if window_secs is not None else None
-
-        with self._lock:
-            self._ensure_loaded_locked()
-            entries = list(self._entries.values())
-
-        pending: List[SessionEntry] = []
-        for entry in entries:
-            if not entry.resume_pending or entry.suspended or entry.origin is None:
-                continue
-            if allowed_reasons is not None and entry.resume_reason not in allowed_reasons:
-                continue
-            if window is not None and window > 0:
-                marker = entry.last_resume_marked_at or entry.updated_at
-                if marker is not None and (current - marker).total_seconds() > window:
-                    continue
-            pending.append(entry)
-
-        pending.sort(key=lambda entry: entry.last_resume_marked_at or entry.updated_at)
-        return pending
-
     def prune_old_entries(self, max_age_days: int) -> int:
         """Drop SessionEntry records older than max_age_days.