Wire the security threat model for agent discoverability (AGENTS.md + SECURITY.md); rename draft-THREAT-MODEL.md -> THREAT_MODEL.md

[potiuk]

This is a proposal for the Jackrabbit PMC to review — please correct, reject, or discuss as needed. Nothing here is a requirement; the maintainer is the decision-maker.

Thanks for merging the threat-model draft, Angela. Two small follow-ups make it mechanically discoverable to automated security scanners (and to any agent that follows the conventional file layout):

1. Wire the discoverability chain. This adds AGENTS.md (a ## Security pointer) and SECURITY.md, so the conventional AGENTS.md → SECURITY.md → THREAT_MODEL.md chain resolves. An automated scanner mechanically follows that chain to locate the model; without it, the model file is present but the scanner can't find it — discoverability is the one hard gate before a scan can run. (On jackrabbit-oak this appends a ## Security pointer to the existing AGENTS.md rather than creating a new one.)

2. Rename draft-THREAT-MODEL.md → THREAT_MODEL.md (optional proposal). Two reasons:

• It's no longer a draft. The PMC has reviewed and merged it, so the draft- prefix now reads as "not yet adopted" when it is, in fact, the project's adopted model — the name undersells it.
• THREAT_MODEL.md is the conventional, discoverable name. It's what the AGENTS.md → SECURITY.md → model chain and the scanners look for by default, and it matches the layout used across the other Apache repos in this effort (e.g. apache/solr). The rename also normalizes the separator (THREAT-MODEL → THREAT_MODEL).

If you'd rather keep the draft- name (or pick a different one), just say so — the rename is the optional part; the wiring is the substantive fix, and we'll point SECURITY.md at whatever filename you prefer.

Context: the ASF Security team is preparing the project for an automated agentic security scan. This PR changes no model content — only the file name and the discoverability wiring around it.