GHSA-JC3J-X6PG-4HMV Algernon: Host header path traversal in --domain mode reads files and runs Lua from parent dir

Summary When algernon is started with --domain or --letsencrypt, which silently turns on --domain at engine/flags.go:372, the request handler resolves the served directory by joining the configured --dir with the value of the client-supplied Host header. The join is performed by filepath.Join wit...