[Improvement] Add AGENTS.md + SECURITY.md to make the security model discoverable

AGENTS.md

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+<!--
+SPDX-License-Identifier: Apache-2.0
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    https://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+-->
+
+# Agent Guide for dolphinscheduler
+
+This file is read by automated agents (security scanners, code
+analyzers, AI assistants) operating on this repository.
+
+## Security
+
+Security model: [SECURITY.md](./SECURITY.md)
+
+Agents that scan this repository should consult `SECURITY.md` and the
+threat model it links before reporting issues.
+
+DolphinScheduler's security model defines user roles, trust boundaries, and known non-findings.

SECURITY.md

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+# Security Policy
+
+## Reporting a Vulnerability
+
+`apache/dolphinscheduler` follows the [Apache Software Foundation security process](https://www.apache.org/security/). Please report suspected
+vulnerabilities privately to `security@apache.org`; do not open public
+GitHub issues or pull requests for security reports.
+
+## Threat Model
+
+What the project treats as in scope and out of scope, the security
+properties it provides and disclaims, the adversary model, and how
+findings are triaged are documented in https://github.com/apache/dolphinscheduler/blob/dev/docs/docs/en/contribute/join/security-model.md.