Add draft security threat model (THREAT_MODEL.md + SECURITY.md + AGENTS.md)

AGENTS.md

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+<!-- SPDX-License-Identifier: Apache-2.0
+     https://www.apache.org/legal/release-policy.html -->
+
+# Agent Guide for drill
+
+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.

SECURITY.md

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+<!-- SPDX-License-Identifier: Apache-2.0
+     https://www.apache.org/legal/release-policy.html -->
+
+# Security Policy
+
+## Reporting a Vulnerability
+
+`apache/drill` 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 [THREAT_MODEL.md](./THREAT_MODEL.md).