Add AGENTS.md so the threat model is discoverable by agents

[potiuk]

What

Adds an AGENTS.md at the repository root pointing at SECURITY.md, and adds it to .ratignore alongside SECURITY.md.

Why

CALCITE-7605 landed the threat model at site/_docs/security_threat_model.md together with a SECURITY.md that links to it. That part is complete and reads well.

What is missing is the entry point. Automated agents — security scanners, code analyzers, coding assistants — locate a project's security model by following a fixed chain:

AGENTS.md  ->  SECURITY.md  ->  the threat model

apache/calcite has the second and third hops but not the first, so the chain cannot be walked mechanically and an agent starting from a clone has no way to find the model on its own.

This matters concretely for the ASF Security team's scanning programme: we refuse to run a scan against a project whose model is not mechanically discoverable, because without the model in context the false-positive rate is more than a PMC can reasonably triage. This PR closes that gap for Calcite.

What is in the file

No new claims — everything restates the merged model:

• A pointer to SECURITY.md and onward to site/_docs/security_threat_model.md.
• The framing that Calcite is embedded, opens no socket, and has no authentication or authorization of its own.
• The two rules that carry most of the triage weight: the surprising vs unsurprising class loading gate (only SPI implementations are instantiated by name, via Class.forName(name, false, loader) plus isAssignableFrom, across the enumerated SPI positions), and the pushed-down SQL split (generated SQL reaching a configured backend is not a vulnerability; a pushdown bug reading beyond the configured schemas is P4 and is one).
• Signposts to the model's "Not a vulnerability" and "Downstream responsibilities" sections.

Notes for reviewers

• Docs only. No production code is touched.
• .ratignore: AGENTS.md carries no license header, so it is excluded exactly as CALCITE-7605 excluded SECURITY.md. Happy to add a header and drop the .ratignore line instead if the PMC prefers that direction.
• No JIRA reference in the title. CALCITE-7605 is resolved, and I did not want to reopen it or file a ticket on the project's behalf unprompted. If Calcite would like a JIRA for this, say the word and I will file one and retitle the PR to match the [CALCITE-NNNN] convention.
• I deliberately did not link the rendered site URL for the threat model, since it does not resolve yet — the in-repo path is what an agent follows anyway.

Opened by the ASF Security team as part of the Frontier Model Preparation scan pre-flight for Calcite, following the request on the PMC's [GLASSWING] thread. Review, change, or close as the PMC sees fit — the document is the project's.

🤖 Generated with Claude Code

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[rubenada]

Thanks @potiuk for the patch!