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 Tapestry
+
+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. Tapestry is a component-based web
+framework: the untrusted web client is the adversary, while application
+pages/components/handlers and configuration are trusted. The single most
+load-bearing mechanism is the HMAC-protected serialized client state
+(deserialization is gated by `tapestry.hmac-passphrase`); the model also covers
+default output escaping, asset access control, client whitelisting, and
+secure-link enforcement.

SECURITY.md

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+# Security Policy
+
+## Reporting a Vulnerability
+
+Apache Tapestry follows the [Apache Software Foundation security process](https://www.apache.org/security/).
+Please report suspected vulnerabilities **privately** to `security@apache.org` (the Tapestry PMC is reachable
+at `private@tapestry.apache.org`). Do **not** open public GitHub issues or pull requests for security reports.
+
+## Threat Model
+
+What Tapestry treats as in/out of scope, the security properties it provides and disclaims (HMAC-gated
+serialized-state deserialization, default output escaping, asset access control, client whitelisting,
+secure-link enforcement), the adversary model (the untrusted web client vs. the trusted application
+developer/operator), and how findings are triaged are documented in [THREAT_MODEL.md](./THREAT_MODEL.md).