fix(followups): F3 - .mam.env BOM, repeated keys and a missing MAM_ENV_FILE

PR #43 review (grok):
(a) A UTF-8 byte-order mark at the start of the file made the first line an
    "invalid key", so its setting was dropped. The one parser (envfile.py,
    used by the wrapper and mqtt_common) now reads with utf-8-sig.
(b) A key given twice keeps its first value (a later line does not replace
    it). The behaviour is unchanged; .mam.env.example's FORMAT section and
    envfile.py's docstring now say so.
(c) MESSAGING.md said a MAM_ENV_FILE naming no file logs an error and makes
    connections fail closed. Both loaders skip it silently and refuse the
    ambient search; a file that exists but cannot be read is the error
    (EnvFileError, one ERROR line and rc 1). The text now says that.

Tests A7: BOM in both loaders, first-value-wins in both loaders plus the
FORMAT text, and a missing explicit file loading neither it nor the ambient
.mam.env without an error plus the MESSAGING text (all three fail at 0051fd6).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HCGpJXhSZasJLU38ksDghe

.mam.env.example

@@ -21,7 +21,9 @@
 #   - `export KEY=...`, a key that is not [A-Za-z_][A-Za-z0-9_]*, a line
 #     without `=`, a line with a NUL byte, and bash's own variables (IFS, PS4,
 #     RANDOM, BASH_*, ...) are skipped with a warning naming the line number;
+#   - a UTF-8 byte-order mark at the start of the file is ignored;
 #   - a variable already set in the environment wins over this file;
+#   - a key given twice keeps its FIRST value (a later line does not replace it);
 #   - a file that exists but cannot be read is an error (the wrapper exits 1).
 #
 # SECURITY: never put real secrets in this template. Secret-bearing vars use a