feat: add pneumatube gateway bridging smolmail and SMTP via LMTP

tests/test_email_to_smol.py

@@ -0,0 +1,20 @@
+from email.message import EmailMessage
+
+from pneumatube.email_to_smol import EmailToSmolBridge, _single_line
+
+
+def test_single_line_strips_crlf():
+    assert _single_line("alice@example.org\r\nAccept: forged") == "alice@example.org  Accept: forged"
+
+
+def test_extract_text_plain_message():
+    message = EmailMessage()
+    message.set_content("hello there")
+    assert EmailToSmolBridge.extract_text(message) == "hello there\n"
+
+
+def test_extract_text_multipart_prefers_plain_part():
+    message = EmailMessage()
+    message.set_content("plain body")
+    message.add_alternative("<p>html body</p>", subtype="html")
+    assert "plain body" in EmailToSmolBridge.extract_text(message)