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() {

tests/unit/02_config.sh

@@ -76,3 +76,25 @@ echo 'bogus_key = 1' >> "$CFG/config"
 sb --print-policy --net none
 assert_contains "unknown key: bogus_key" "unknown key warns"
 assert_rc 0 "unknown key is not fatal"
+
+t_name "an allow file may end on a comment or a blank line"
+# Regression: cfg_read_allowlist() ended on its `[[ -n "$line" ]]`, so a file whose last
+# line yields no target returned 1. load_config() calls it in a non-final position, so under
+# `set -e` every run died with exit 1 and *zero* bytes of output -- --dry-run and
+# --print-policy included. The test above ends its file on "host  # trailing comment", which
+# yields a target and so never exercised this.
+for last in '# a trailing comment' '' '   '; do
+  printf '# user-wide allowlist\napi.openai.com\n%s\n' "$last" > "$CFG/allow"
+  printf 'pypi.org\n%s\n'                                      "$last" > "$REPO/.ai-sandbox/allow"
+  sb --print-policy --net allow
+  assert_rc 0 "user allow file ending on [$last] is not fatal"
+  assert_contains "api.openai.com" "user target survives a file ending on [$last]"
+  assert_contains "pypi.org"       "repo target survives a file ending on [$last]"
+done
+# The fix must not trade one silence for another: with no target anywhere, the run still
+# refuses, and says why.
+printf '# nothing but a header here\n' > "$CFG/allow"
+printf '# nothing but a header here\n' > "$REPO/.ai-sandbox/allow"
+sb_timeout 20 --print-policy --net allow
+assert_rc 1 "a header-only allow file still refuses to start"
+assert_contains "no egress allowlist configured" "and refuses out loud"