GHSA-2jcc-mxv7-p3f9

## Summary

From **v1.13.2** through **v1.18.0**, oasdiff did not enforce `--allow-external-refs=false` (library: `openapi3.Loader.IsExternalRefsAllowed = false`) when loading a spec from a **git revision** (the `rev:path` form, e.g. `main:openapi.yaml`). External `$ref`s were resolved on that load path even when external refs were explicitly disabled, so the mitigation silently did not apply there.

## Impact

A caller who set `--allow-external-refs=false` *specifically to safely process untrusted specs* remained exposed — on the git-revision load path only — to:

- **SSRF** via `$ref: "http://<internal-host>/…"`, and - **Local file reads** via `$ref: "/path"` or `file://`.

Affected callers:

- **CLI:** `oasdiff diff main:openapi.yaml HEAD:openapi.yaml --allow-external-refs=false` (and `breaking` / `changelog` / `summary`, and the `git-diff-driver`) run over untrusted spec content. - **Go library consumers** of `github.com/oasdiff/oasdiff/load` that set `IsExternalRefsAllowed = false` and load from a git-revision source via `load.NewSpecInfo`.

The file and URL load paths correctly enforced the setting; only the git-revision path was affected. Callers that left external refs at the default (`true`) are not in scope for *this* advisory.

## Patches

**v1.18.1** enforces the external-refs policy on the git-revision path (so `--allow-external-refs=false` now blocks external `$ref`s there) and returns a dedicated exit code (`123`) when an external `$ref` is refused.

## Workarounds

- Upgrade to **v1.18.1**, or - Avoid the git-revision input form when processing untrusted specs with external refs disabled.

## Notes

- Introduced in **v1.13.2** (#832, which added `$ref`-chain resolution on the git-revision path); fixed in **v1.18.1** (#974, #975). - The permissive **default** (`allow-external-refs: true`) and its zero-interaction exposure in CI via the GitHub Action is tracked separately in GHSA-fhj3-7267-7vv5 (oasdiff-action).