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/db_tools.py

@@ -752,7 +752,9 @@ class MemoTool(Tool):
 
         memo_id = kwargs.get("memo_id")
         if memo_id:
-            if self.memo_db.complete_memo(memo_id, force_complete=True):
+            # M5：真实物理删除（此前调 complete_memo 只标记完成，行不删除，
+            # 重复任务被推进而非移除，stats 计数失真）
+            if self.memo_db.delete_memo(memo_id):
                 return self.format_result(True, f"任务 #{memo_id} 已删除")
             else:
                 return f"❌ 无法删除任务 #{memo_id}"