Stop a trailing comment in an allow file killing every run

cfg_read_allowlist() ended on `[[ -n "$line" ]] && CFG_allow+=(...)`, so a file whose
last line is a comment or blank returned 1 from the loop. load_config() calls it twice
in a non-final position, so under `set -e` every run died with exit 1 and no output at
all -- --dry-run and --print-policy included, with nothing to explain it.

Reading a file now always succeeds. An allowlist with nothing in it stays
validate_opts()'s call to refuse, with the message that says why.

LLM-Generated: true

bin/ai-sandbox

@@ -348,6 +348,13 @@ cfg_read_allowlist() { # one target per line
     line="$(trim "${line%%#*}")"
     [[ -n "$line" ]] && CFG_allow+=("$line")
   done < "$file"
+  # A loop returns the status of the last command its body ran, and here that is the `&&`
+  # above: a file whose last line is a comment or blank would return 1. load_config() calls
+  # this twice in a non-final position, so under `set -e` every run would die with exit 1
+  # and no output at all -- previews included. A blank last line is not an error, so reading a
+  # file always succeeds; an allowlist with nothing in it is validate_opts()'s call to
+  # refuse, with a message, rather than a silent death here.
+  return 0
 }
 
 load_config() {