For c2d31a5e59: docs/gateway/security/index.md "External content
special-token sanitization" section already mentions the outbound
sanitizer with `<tool_call>` and `<function_calls>` examples, but it
predates the new internal-runtime-scaffolding stripping that targets
`<system-reminder>` and `<previous_response>` tags. Adds those two tags
as explicit examples and notes the final channel delivery boundary so
operators reading the security page see the same coverage exposed by
the c2d31a5e59 sanitizer.