fix(issue-26): review - an unreadable env file is an error, NUL lines are skipped, integers are ASCII in every locale, empty flags are refused

Merge-workspace review of PR #43 (codex R1-R4, claude R1/R2).

- R1: envfile.parse raises EnvFileError when the file exists but cannot be
  read or decoded as a whole; `--shell` prints the cause and exits 1, so the
  wrapper stops (rc 1, as before #26) instead of running on defaults, and
  mqtt_common's entry points stop on the same error. A bad single line is
  still a warning. The wrapper's message no longer claims a read failure
  when the cause is elsewhere ("cannot load ... see the line above").
- R2: lib.sh mam_int spells out the ASCII blanks and digits instead of
  [[:space:]]/[0-9], which follow the locale: U+2003 30 U+2003 was 30 in the
  wrapper and invalid in the runner under en_US.UTF-8. The test oracle
  _expected uses the same ASCII rule.
- R3: an integer flag given explicitly with an empty value is refused like
  any other invalid value; only an absent flag keeps its default.
- R4: a line holding a NUL byte is skipped with a warning by both loaders;
  before, the wrapper read "leftright" and the Python loader died.
- .mam.env.example states the format: no expansion, `export` and bash's own
  variables skipped, a trailing `# ...` is part of the value, the environment
  wins, an unreadable file is an error.

Tests: unreadable (0000) and invalid-UTF-8 files stop the wrapper and the
Python loader; a NUL line is skipped by both; Unicode blanks/digits are
invalid for mam_int, _positive_int, the wrapper and the runner under C and
en_US.UTF-8; explicit empty values for the five integer flags are refused.
All of them fail on b58edac.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VCgu9Fa9iT7TbJUSzdq4Ff

.agents/skills/multi-agent-mux-delegate-job/multi-agent-mux-delegate-job

@@ -41,7 +41,7 @@ fi
 if [[ -n "$TARGET_ENV" && -f "$TARGET_ENV" ]]; then
   _env_exports="$(SCRIPT_DIR="$SCRIPT_DIR" REPO_ROOT="$REPO_ROOT" TARGET_ENV="$TARGET_ENV" \
     "${DELEGATE_JOB_PYTHON:-python3}" -I "$SCRIPT_DIR/../lib_py/delegation/envfile.py" --shell "$TARGET_ENV")" \
-    || { echo "ERROR: cannot read $TARGET_ENV" >&2; exit 1; }
+    || { echo "ERROR: cannot load $TARGET_ENV (see the line above)" >&2; exit 1; }
   eval "$_env_exports"; unset _env_exports
 fi
 
@@ -164,6 +164,7 @@ COUNTERPART_ROLE_EXPLICIT=0
 PARENT_JOB=""
 
 parse_opts() {
+  _INT_FLAGS_GIVEN=""
   while [[ $# -gt 0 ]]; do
     case "$1" in
       --agent) AGENT="$2"; shift 2;;
@@ -171,11 +172,11 @@ parse_opts() {
       --prompt-file) PROMPT_FILE="$2"; shift 2;;
       --workdir) WORKDIR="$2"; shift 2;;
       --agent-session) AGENT_SESSION="$2"; shift 2;;
-      --timeout) TIMEOUT="$2"; shift 2;;
-      --idle-timeout) IDLE_TIMEOUT="$2"; shift 2;;
-      --heartbeat) HEARTBEAT_SEC="$2"; shift 2;;
-      --heartbeat-misses) HEARTBEAT_MISSES="$2"; shift 2;;
-      --ack-timeout) ACK_TIMEOUT="$2"; shift 2;;
+      --timeout) TIMEOUT="$2"; _INT_FLAGS_GIVEN+=" TIMEOUT"; shift 2;;
+      --idle-timeout) IDLE_TIMEOUT="$2"; _INT_FLAGS_GIVEN+=" IDLE_TIMEOUT"; shift 2;;
+      --heartbeat) HEARTBEAT_SEC="$2"; _INT_FLAGS_GIVEN+=" HEARTBEAT_SEC"; shift 2;;
+      --heartbeat-misses) HEARTBEAT_MISSES="$2"; _INT_FLAGS_GIVEN+=" HEARTBEAT_MISSES"; shift 2;;
+      --ack-timeout) ACK_TIMEOUT="$2"; _INT_FLAGS_GIVEN+=" ACK_TIMEOUT"; shift 2;;
       --validate) VALIDATE="$2"; shift 2;;
       --job) JOB_ID="$2"; shift 2;;
       --registry-dir) REGISTRY_DIR="$2"; shift 2;;
@@ -191,10 +192,13 @@ parse_opts() {
       *) echo "unknown option: $1" >&2; usage; exit 1;;
     esac
   done
-  # Issue #26: a flag value that is not a positive decimal integer is refused.
+  # Issue #26: a flag value that is not a positive decimal integer is refused -
+  # an explicitly given empty value too (review R3); only an absent flag keeps
+  # its default.
   local _flag _var _n
   for _flag in timeout:TIMEOUT idle-timeout:IDLE_TIMEOUT heartbeat:HEARTBEAT_SEC heartbeat-misses:HEARTBEAT_MISSES ack-timeout:ACK_TIMEOUT; do
-    _var="${_flag#*:}"; [ -n "${!_var}" ] || continue
+    _var="${_flag#*:}"
+    [ -n "${!_var}" ] || [[ " $_INT_FLAGS_GIVEN " == *" $_var "* ]] || continue
     _n=$(mam_int "${!_var}" 1) || { echo "ERROR: --${_flag%%:*} ${!_var}: not a positive decimal integer" >&2; exit 1; }
     printf -v "$_var" '%s' "$_n"
   done