fix(agent): persist user message before running turn loop

The existing runtime_checkpoint mechanism preserves the in-flight
assistant/tool state if the process dies mid-turn, but the triggering
user message is only written to session history at the end of the turn
via _save_turn(). If the worker is killed (OOM, SIGKILL, a self-
triggered systemctl restart, container eviction, etc.) before the turn
completes, the user's message is silently lost: on restart, the session
log only shows the interrupted assistant turn without any record of
what the user asked. Any recovery tooling built on top of session logs
cannot reply because it has no prompt to reply to.

This patch appends the incoming user message to the session and flushes
it to disk immediately after the session is loaded and before the agent
loop runs, then adjusts the _save_turn skip offset so the final
persistence step does not duplicate it.

Limited to textual content (isinstance(msg.content, str)); list-shaped
content (media blocks) still flows through _save_turn's sanitization at
end of turn, preserving existing behavior for those cases.

nanobot/agent/loop.py

@@ -693,6 +693,23 @@ class AgentLoop:
                 )
             )
 
+        # Persist the triggering user message immediately, before running the
+        # agent loop. If the process is killed mid-turn (OOM, SIGKILL, self-
+        # restart, etc.), the existing runtime_checkpoint preserves the
+        # in-flight assistant/tool state but NOT the user message itself, so
+        # the user's prompt is silently lost on recovery. Saving it up front
+        # makes recovery possible from the session log alone.
+        user_persisted_early = False
+        if isinstance(msg.content, str) and msg.content.strip():
+            from datetime import datetime as _dt
+            session.messages.append({
+                "role": "user",
+                "content": msg.content,
+                "timestamp": _dt.now().isoformat(),
+            })
+            self.sessions.save(session)
+            user_persisted_early = True
+
         final_content, _, all_msgs, stop_reason, had_injections = await self._run_agent_loop(
             initial_messages,
             on_progress=on_progress or _bus_progress,
@@ -708,7 +725,9 @@ class AgentLoop:
         if final_content is None or not final_content.strip():
             final_content = EMPTY_FINAL_RESPONSE_MESSAGE
 
-        self._save_turn(session, all_msgs, 1 + len(history))
+        # Skip the already-persisted user message when saving the turn
+        save_skip = 1 + len(history) + (1 if user_persisted_early else 0)
+        self._save_turn(session, all_msgs, save_skip)
         self._clear_runtime_checkpoint(session)
         self.sessions.save(session)
         self._schedule_background(self.consolidator.maybe_consolidate_by_tokens(session))