feat: auto-continue interrupted agent work after gateway restart (#4493)

When the gateway restarts mid-agent-work, the session transcript ends
on a tool result the agent never processed. Previously, the user had
to type 'continue' or use /retry (which replays from scratch, losing
all prior work).

Now, when the next user message arrives and the loaded history ends
with role='tool', a system note is prepended:

  [System note: Your previous turn was interrupted before you could
  process the last tool result(s). Please finish processing those
  results and summarize what was accomplished, then address the
  user's new message below.]

This is injected in _run_agent()'s run_sync closure, right before
calling agent.run_conversation(). The agent sees the full history
(including the pending tool results) and the system note, so it can
summarize what was accomplished and then handle the user's new input.

Design decisions:
- No new session flags or schema changes — purely detects trailing
  tool messages in the loaded history
- Works for any restart scenario (clean, crash, SIGTERM, drain timeout)
  as long as the session wasn't suspended (suspended = fresh start)
- The user's actual message is preserved after the note
- If the session WAS suspended (unclean shutdown), the old history is
  abandoned and the user starts fresh — no false auto-continue

Also updates the shutdown notification message from 'Use /retry after
restart to continue' to 'Send any message after restart to resume
where it left off' — which is now accurate.

Test plan:
- 6 new auto-continue tests (trailing tool detection, no false
  positives for assistant/user/empty history, multi-tool, message
  preservation)
- All 13 restart drain tests pass (updated /retry assertion)

gateway/run.py

@@ -1405,7 +1405,7 @@ class GatewayRunner:
         action = "restarting" if self._restart_requested else "shutting down"
         hint = (
             "Your current task will be interrupted. "
-            "Use /retry after restart to continue."
+            "Send any message after restart to resume where it left off."
             if self._restart_requested
             else "Your current task will be interrupted."
         )
@@ -8450,6 +8450,21 @@ class GatewayRunner:
             if _msn:
                 message = _msn + "\n\n" + message
 
+            # Auto-continue: if the loaded history ends with a tool result,
+            # the previous agent turn was interrupted mid-work (gateway
+            # restart, crash, SIGTERM).  Prepend a system note so the model
+            # finishes processing the pending tool results before addressing
+            # the user's new message.  (#4493)
+            if agent_history and agent_history[-1].get("role") == "tool":
+                message = (
+                    "[System note: Your previous turn was interrupted before you could "
+                    "process the last tool result(s). The conversation history contains "
+                    "tool outputs you haven't responded to yet. Please finish processing "
+                    "those results and summarize what was accomplished, then address the "
+                    "user's new message below.]\n\n"
+                    + message
+                )
+
             _approval_session_key = session_key or ""
             _approval_session_token = set_current_session_key(_approval_session_key)
             register_gateway_notify(_approval_session_key, _approval_notify_sync)