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

tests/test_issue26_envfile.py

@@ -405,3 +405,47 @@ def test_a6_the_file_cannot_replace_the_wrappers_own_variables(tmp_path):
     assert not (tmp_path / "pwned-lib").exists()
     got = _wrapper_values(_env(tmp_path, f), ["SCRIPT_DIR", "TARGET_ENV"], tmp_path)
     assert _split(got.stdout) == {"SCRIPT_DIR": str(WRAPPER.parent), "TARGET_ENV": str(f)}
+
+
+# ---------------------------------------------------------------------------
+# A7 - follow-ups F3 (PR #43 review grok)
+# ---------------------------------------------------------------------------
+
+def test_a7_a_leading_bom_is_not_part_of_the_first_key(tmp_path):
+    """An editor's byte-order mark used to turn the first line into
+    "invalid key" and drop the first setting, in both loaders."""
+    f = tmp_path / "bom.env"
+    f.write_bytes(b"\xef\xbb\xbfFIRST=one\nSECOND=two\n")
+    env = _env(tmp_path, f)
+    res = _wrapper_values(env, ["FIRST", "SECOND"], tmp_path)
+    assert res.returncode == 0, res.stderr
+    assert _split(res.stdout) == {"FIRST": "one", "SECOND": "two"}
+    assert "skipped" not in res.stderr, res.stderr
+    assert _python_values(env, ["FIRST", "SECOND"]) == {"FIRST": "one", "SECOND": "two"}
+    assert envfile.parse(str(f)) == ([("FIRST", "one"), ("SECOND", "two")], [])
+
+
+def test_a7_a_repeated_key_keeps_its_first_value_as_documented(tmp_path):
+    f = tmp_path / "dup.env"
+    f.write_text("DUP=first\nDUP=second\n")
+    env = _env(tmp_path, f)
+    res = _wrapper_values(env, ["DUP"], tmp_path)
+    assert res.returncode == 0, res.stderr
+    assert _split(res.stdout) == {"DUP": "first"} == _python_values(env, ["DUP"])
+    fmt = (REPO_ROOT / ".mam.env.example").read_text().split("# SECURITY:")[0]
+    assert "a key given twice keeps its FIRST value" in fmt, "document it in the FORMAT section"
+
+
+def test_a7_a_missing_explicit_file_loads_nothing_and_says_nothing(tmp_path):
+    """MAM_ENV_FILE naming no file: no error, and no fallback to the file the
+    ambient search would find - in both loaders, and so in MESSAGING.md."""
+    (tmp_path / ".mam.env").write_text("PROBE_AMBIENT=found\n")
+    absent = tmp_path / "absent.env"
+    env = _env(tmp_path, absent)
+    res = _wrapper_values(env, ["PROBE_AMBIENT"], tmp_path)
+    assert res.returncode == 0, res.stderr
+    assert _split(res.stdout) == {"PROBE_AMBIENT": "<unset>"}
+    assert "ERROR" not in res.stderr and str(absent) not in res.stderr, res.stderr
+    assert _python_values(env, ["PROBE_AMBIENT"]) == {"PROBE_AMBIENT": "<unset>"}
+    doc = next(l for l in (REPO_ROOT / "MESSAGING.md").read_text().splitlines() if "Explicit Override (`MAM_ENV_FILE`)" in l)
+    assert "does not exist, it is skipped silently" in doc and "an error is logged" not in doc, doc