fix(aibox): batch H audit fixes - subagent cancel propagation, scheduler timeout, exec process-group reaping

Fourth audit round (see docs/reports/aibox_audit_20260817.md §11):
- H1: shared parent cancel event no longer cleared on subagent first think
  (N5 regression); parent-already-cancelled skips subagent start
- M1: /reload_prompts moved off event loop (run_in_executor, N3 sibling)
- M2: skill --help scan prewarmed before serving connections
- M3: scheduler task-level timeout (scheduler.task_timeout, default 600s)
- M4: get_recent_memories(None) capped instead of full-table load
- M5: memo delete now physically removes rows (was complete-only)
- M6: PromptManager.reload builds new list then atomically swaps
- L1-L12: read_file arg coercion, proxy save failure reported, SSE
  utf-8 tolerance, persona realpath check, truncation char boundary,
  compressor div0, skill path quoting, ask_user required, POSIX
  process-group kill (pgid captured at Popen), web scheme whitelist,
  registry redact substring match, Bearer redaction in failover logs

aibox/tools/proxy_tools.py

@@ -146,7 +146,7 @@ class SetProxyTool(Tool):
             check_detail = detail
 
         config = ConfigManager()
-        config.set_proxy(host, port, username, password, password_env, enabled, save)
+        saved = config.set_proxy(host, port, username, password, password_env, enabled, save)
         proxy_info = config.get_proxy_info()
         if proxy_info.get("password_env"):
             pw_source = "环境变量 " + proxy_info["password_env"]
@@ -159,10 +159,13 @@ class SetProxyTool(Tool):
             "服务器": f"{proxy_info['host']}:{proxy_info['port']}",
             "认证": "有" if proxy_info.get("username") else "无",
             "密码来源": pw_source,
-            "持久化": "已保存" if save else "仅内存"
+            "持久化": "已保存" if saved else "保存失败（仅本次会话有效，重启后丢失）"
         }
         if check_detail:
             data["连通性"] = check_detail
+        # L2：落盘失败（磁盘只读等）如实报告，不再静默报"已保存"
+        if save and not saved:
+            return self.format_result(False, "代理已配置但保存失败（仅本次会话有效，重启后丢失）", data)
         return self.format_result(True, "代理设置成功", data)