Add SECURITY.md + AGENTS.md Security section for scan-agent discoverability

[potiuk]

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

This adds a SECURITY.md to the repo root and a Security section to the existing AGENTS.md so an automated scan agent can mechanically discover the project's security model via the conventional AGENTS.md → SECURITY.md → model URL chain. The chain terminates at the existing https://spark.apache.org/docs/latest/security.html page — nothing about the model content itself changes.

Context: the ASF Security team is preparing the project for an automated agentic security scan we're piloting. Such scans refuse to run if the model isn't discoverable by that path (refusing upfront beats wasting PMC reviewer cycles on a noise-heavy run against an unknown model). Discoverability is the one hard gate; everything else is suggestion. The Security team has reached out separately on the PMC's private list with the program details; this PR is the public-facing repo piece.

The Security team uses threat-model-producer as the rubric for what a complete model looks like — but this PR is just the link; the existing security.html content is accepted as the model.

After this lands on master, the same two files would need to be on branch-3.5 for the second scan target — happy to open a cherry-pick PR for that, or leave it to the PMC.

Questions / pushback welcome. Happy to adjust the wording or move the section if the project has a house style.