Add SECURITY.md + AGENTS.md pointing at the published security model

[potiuk]

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

This adds two small files so that an automated scanning agent can mechanically discover the project's existing security model through the conventional AGENTS.md → SECURITY.md chain:

• SECURITY.md — points at the published model at https://avro.apache.org/project/security/, which remains the single source of truth. Nothing is duplicated or restated here; the file is a pointer.
• AGENTS.md — a Security section linking to SECURITY.md.

No existing content is changed — both files are new, and the security model itself is untouched.

Context

The Apache Security team is preparing the project for an automated agentic security scan we're piloting. Such scans anchor to a project's own threat model, and refuse to run when the model can't be found from the repository — refusing upfront beats spending PMC reviewer time triaging a noise-heavy report produced against no model at all.

Avro is in the good case here: the project already has a published security model, so nobody needs to write one. We read it against our rubric and it is usable as-is — it is explicit about what the library is responsible for, that transport is outside the library's scope, that avoiding leaks into side channels such as log files is a stated non-goal, that schema parsing itself is safe while SPIs remain a trust decision, and it places four numbered duties on applications using Avro. That is more than many projects start with.

The only gap was mechanical: there is no AGENTS.md or SECURITY.md at the repository root, so the discovery chain doesn't exist. This PR is that chain and nothing more.

This was requested on the PMC's private list — Martin Grigorov asked us to go ahead on behalf of the Avro PMC.

Not in this PR

Separately, and explicitly not blocking anything, the PMC has invited us to propose deepening the published model. The sections most useful to a scanner are an enumerated adversary model, a "known non-findings" list (the single biggest lever for cutting false positives in a report), and triage dispositions. Those touch the website content rather than this repository, so they'll come as a separate proposal.

Questions and pushback welcome — happy to adjust wording or file placement to match project house style.