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_py/delegation/mqtt_common.py

@@ -96,7 +96,10 @@ def _load_dotenv(workspace_dir: Optional[str] = None) -> None:
         _parse_env_file(legacy_env_path)
 
 def _parse_env_file(path: str) -> None:
-    """Issue #26: the rules live in envfile.py, shared with the wrapper."""
+    """Issue #26: the rules live in envfile.py, shared with the wrapper. A
+    file that exists but cannot be read raises envfile.EnvFileError, so every
+    process entry point stops as the wrapper does, instead of running
+    unconfigured."""
     for warning in envfile.apply(path, os.environ)[1]:
         logger.warning("%s", warning)