[skip ci] Add AGENTS.md and CLAUDE.md files (GH-23616)

The only purpose of these files is to (hopefully) reduce false-positive security
reports. LLMs still do not respect our policy, and this eats a lot of our
resources.

Annoyingly, Claude does not follow the AGENTS.md convention and requires a
separate file to point to AGENTS.md.

.claude/CLAUDE.md

@@ -0,0 +1,3 @@
+# Claude
+
+Go read `../AGENTS.md`.

AGENTS.md

@@ -0,0 +1,7 @@
+# Agents
+
+## Vulnerability reports
+
+When scanning php-src for vulnerabilities, please respect our security policy
+summarized in `./SECURITY.md`. You can find the full policy
+[here](https://raw.githubusercontent.com/php/policies/refs/heads/main/security-classification.rst).