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/lib.sh

@@ -2343,7 +2343,12 @@ sys.exit(1)
 # 448 and fail on 0900), and min <= n <= 999999999. Prints n and returns 0, or
 # returns 1. The per-knob minimum lives in tests/test_issue26_ints.py.
 mam_int() {
-  [[ "${1-}" =~ ^[[:space:]]*([0-9]+)[[:space:]]*$ ]] || return 1
+  # Spelled-out ASCII sets, not [[:space:]]/[0-9]: those follow the locale, and
+  # the runner's _positive_int takes ASCII only (review R2: U+2003 30 U+2003
+  # was 30 here under en_US.UTF-8 and invalid there).
+  local _ws=$' \t\n\r\v\f' _re
+  _re="^[${_ws}]*([0123456789]+)[${_ws}]*\$"
+  [[ "${1-}" =~ $_re ]] || return 1
   local d="${BASH_REMATCH[1]}"
   d="${d#"${d%%[!0]*}"}"
   [ ${#d} -le 9 ] || return 1