MINOR: Add draft threat model + SECURITY.md + AGENTS.md for security-model discoverability

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 Apache Kafka
+
+This file is read by automated agents (security scanners, code analyzers,
+AI assistants) operating on this repository.
+
+## Security
+
+Security model: [SECURITY.md](./SECURITY.md) -> [THREAT_MODEL.md](./THREAT_MODEL.md)
+
+Agents that scan this repository should consult `SECURITY.md` and the linked
+`THREAT_MODEL.md` before reporting issues. Kafka is a configurable platform: it
+provides mechanisms (SASL/mTLS authentication, an ACL authorizer, TLS, quotas)
+and many properties are conditional on the operator's configuration. The
+untrusted network client of a broker (or the Connect REST API) is the adversary;
+the operator and trusted cluster peers / metadata quorum are out of model. Note
+the default PLAINTEXT/no-authorizer posture is the classic exposure (see §14).

SECURITY.md

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+# Security Policy
+
+## Reporting a Vulnerability
+
+Apache Kafka follows the [Apache Software Foundation security process](https://www.apache.org/security/).
+Please report suspected vulnerabilities **privately** to `security@apache.org` (the Kafka PMC is reachable
+at `private@kafka.apache.org`). Do **not** open public GitHub issues or pull requests for security reports.
+
+## Threat Model
+
+What Kafka treats as in/out of scope, the security properties it provides and disclaims (authentication via
+SASL/mTLS, ACL authorization, transport security, RPC robustness/quotas), the adversary model (the untrusted
+network client vs. the trusted operator and cluster peers), and how findings are triaged are documented in
+[THREAT_MODEL.md](./THREAT_MODEL.md). Because Kafka is a configurable platform, many properties are
+conditional on the operator's listener/authorizer/TLS configuration.