refactor(application): filter_rules.py 更名 rules.py 并合并 RuleHelper

规则域收敛为单一事实来源 app/application/rules.py：
- filter_rules.py 更名为 rules.py（内置规则集 + RuleParser）
- filter.py 仅含 66 行 RuleHelper（用户规则组配置访问），并入 rules.py
- 同步更新 5 处引用方导入、nettest stub 目标与架构文档
- 兼容映射 app.helper.rule 指向新路径，legacy 导入仍可用

app/agent/tools/impl/_filter_rule_utils.py

@@ -7,9 +7,9 @@ from typing import Any, Dict, Iterable, Optional
 from app.runtime.events import eventmanager
 from app.db.oper.subscribe import SubscribeOper
 from app.db.oper.systemconfig import SystemConfigOper
-from app.application.filter import RuleHelper
-from app.application.filter_rules import RuleParser
-from app.application.filter_rules import BUILTIN_RULE_SET
+from app.application.rules import RuleHelper
+from app.application.rules import RuleParser
+from app.application.rules import BUILTIN_RULE_SET
 from app.schemas import CustomRule, FilterRuleGroup
 from app.schemas.event import ConfigChangeEventData
 from app.schemas.types import EventType, SystemConfigKey