POST /config without an explicit path= always wrote gost.yaml/gost.json to
the process CWD, silently leaving the config loaded via -C untouched. Track
the last file read by ReadFile and default the save target to it, deriving
the output format from the file extension so a JSON config is saved back as
JSON. Falls back to the CWD default when no config file was loaded.
ginuerzh
0132cefaec