feat: add email gateway platform (IMAP/SMTP)

Allow users to interact with Hermes by sending and receiving emails.
Uses IMAP polling for incoming messages and SMTP for replies with
proper threading (In-Reply-To, References headers).

Integrates with all 14 gateway extension points: config, adapter
factory, authorization, send_message tool, cron delivery, toolsets,
prompt hints, channel directory, setup wizard, status display, and
env example.

65 tests covering config, parsing, dispatch, threading, IMAP fetch,
SMTP send, attachments, and all integration points.

tools/send_message_tool.py

@@ -119,6 +119,7 @@ def _handle_send(args):
         "slack": Platform.SLACK,
         "whatsapp": Platform.WHATSAPP,
         "signal": Platform.SIGNAL,
+        "email": Platform.EMAIL,
     }
     platform = platform_map.get(platform_name)
     if not platform:
@@ -185,6 +186,8 @@ async def _send_to_platform(platform, pconfig, chat_id, message, thread_id=None)
         return await _send_slack(pconfig.token, chat_id, message)
     elif platform == Platform.SIGNAL:
         return await _send_signal(pconfig.extra, chat_id, message)
+    elif platform == Platform.EMAIL:
+        return await _send_email(pconfig.extra, chat_id, message)
     return {"error": f"Direct sending not yet implemented for {platform.value}"}
 
 
@@ -283,6 +286,35 @@ async def _send_signal(extra, chat_id, message):
         return {"error": f"Signal send failed: {e}"}
 
 
+async def _send_email(extra, chat_id, message):
+    """Send via SMTP (one-shot, no persistent connection needed)."""
+    import smtplib
+    from email.mime.text import MIMEText
+
+    address = extra.get("address") or os.getenv("EMAIL_ADDRESS", "")
+    password = os.getenv("EMAIL_PASSWORD", "")
+    smtp_host = extra.get("smtp_host") or os.getenv("EMAIL_SMTP_HOST", "")
+    smtp_port = int(os.getenv("EMAIL_SMTP_PORT", "587"))
+
+    if not all([address, password, smtp_host]):
+        return {"error": "Email not configured (EMAIL_ADDRESS, EMAIL_PASSWORD, EMAIL_SMTP_HOST required)"}
+
+    try:
+        msg = MIMEText(message, "plain", "utf-8")
+        msg["From"] = address
+        msg["To"] = chat_id
+        msg["Subject"] = "Hermes Agent"
+
+        server = smtplib.SMTP(smtp_host, smtp_port)
+        server.starttls()
+        server.login(address, password)
+        server.send_message(msg)
+        server.quit()
+        return {"success": True, "platform": "email", "chat_id": chat_id}
+    except Exception as e:
+        return {"error": f"Email send failed: {e}"}
+
+
 def _check_send_message():
     """Gate send_message on gateway running (always available on messaging platforms)."""
     platform = os.getenv("HERMES_SESSION_PLATFORM", "")