X_UI_DIFF_PATH_OUTSIDE
Means
ui.diff was handed a path that does not resolve inside .x/shot/
Typical cause
an absolute path, a .. segment, a sibling directory sharing the prefix's characters, or a symlink under .x/shot/ pointing out of it. The gate is what lets a file-reading tool sit under dev:read: it reads the captures the ui.* tools wrote and nothing else, so a read token cannot reach .env through a tool that says "diff"
Fix
x shot / --json, then pass the image path it answers, relative to the app root