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.

cron/scheduler.py

@@ -103,6 +103,7 @@ def _deliver_result(job: dict, content: str) -> None:
         "slack": Platform.SLACK,
         "whatsapp": Platform.WHATSAPP,
         "signal": Platform.SIGNAL,
+        "email": Platform.EMAIL,
     }
     platform = platform_map.get(platform_name.lower())
     if not platform: