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

.ratignore

@@ -14,6 +14,7 @@
 **/data2.txt
 .idea/vcs.xml
 SECURITY.md
+AGENTS.md
 example/csv/src/test/resources/smoke_test.sql
 
 # TODO: remove when pom.xml files are removed

AGENTS.md

@@ -0,0 +1,43 @@
+# Agent guidance
+
+This file is read by automated agents (security scanners, code analyzers,
+AI assistants) operating on this repository. It points them at the
+human-authored references they should consult before producing output.
+
+## Security
+
+Security model: [SECURITY.md](./SECURITY.md), which links to the project's
+threat model at
+[site/_docs/security_threat_model.md](./site/_docs/security_threat_model.md).
+
+Calcite is an embedded SQL framework, not a server. It opens no socket and
+has no authentication or authorization of its own; the host application
+owns transport, identity, and schema scoping. Read the threat model before
+reporting anything — it is explicit about what it does and does not treat
+as a vulnerability.
+
+Two rules carry most of the triage weight:
+
+- **Surprising vs unsurprising class loading.** A class named through a
+  Calcite SPI position — `schemaFactory`, `parserFactory`, `typeSystem`,
+  `metaTableFactory`, `metaColumnFactory`, `tableFactory`, function
+  classes, `dataSource`, `jdbcDriver`, `model` — is loaded only through
+  that SPI, gated by `Class.forName(name, false, loader)` plus an
+  `isAssignableFrom` check. A class that does not implement the SPI for
+  its position is never instantiated by name. SQL may name SPI classes,
+  but only SPI implementations run, and only through their SPI.
+- **Pushed-down SQL.** The SQL Calcite generates and sends to a backend
+  the operator configured is *not* a vulnerability — the query author can
+  already reach that endpoint through the visible schemas. A pushdown bug
+  that reads *beyond* the configured schemas is P4 and *is* one.
+
+Explicitly not vulnerabilities (see the model's "Not a vulnerability"
+section): the os-adapter running OS commands, the file/CSV/JSON adapters
+reading paths they were configured with, anything requiring a changed
+system property or classpath, a third-party driver's behaviour past the
+connection boundary, and cross-tenant reads that follow from the embedder
+exposing several principals' schemas on one connection.
+
+The model also lists what belongs to the host rather than the library —
+transport and identity, schema scoping, adapter selection, the classpath,
+and whatever a `model` points at — under "Downstream responsibilities".