Add draft project security threat-model document

[potiuk]

What

Adds a v0 THREAT_MODEL.md for Apache Knox, plus the discoverability wiring (SECURITY.md and AGENTS.md), drafted by the ASF Security team for the Knox PMC to review, adjust, and own.

This is path 3 of the Frontier Model Preparation pre-flight — the Knox PMC (Larry McCay, chair) asked on 2026-07-02 for a v0 draft to react to. The document follows the Security team's threat-model rubric: it describes the assumptions Knox makes about its environment and callers, the security properties it upholds and the ones it explicitly disclaims, the operator's responsibilities, and a triage-disposition table for routing a security report.

• THREAT_MODEL.md — the v0 draft (provenance-tagged (documented)/(maintainer)/(inferred); §14 collects the open questions for the PMC, prioritized in waves).
• SECURITY.md — a reporting policy (Knox had none) that links the threat model.
• AGENTS.md — points to SECURITY.md → THREAT_MODEL.md so the model is mechanically discoverable.

For the PMC — highest-leverage open questions

• §14 Q14 — is ungated HeaderPreAuth (trusting an identity header without an mTLS/IP gate) a supported posture, or a misconfiguration the operator must avoid?
• §14 Q24 — ratify "faithful identity assertion" (a client cannot make Knox assert a principal it did not authenticate) as the keystone property.
• §14 Q3 / Q4 — confirm the operator, the federated IdP, and the backend services are out of the adversary model.

The (inferred) claims are the ones needing PMC confirmation; promoting them to (maintainer) as you answer §14 is the fastest path to a ratified model.

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Test Results

32 tests   32 ✅  3s ⏱️
 1 suites   0 💤
 1 files     0 ❌

Results for commit aaff416.