fix(email): allow proactive sends when autoReplyEnabled is false

Previously, `autoReplyEnabled=false` would block ALL email sends,
including proactive emails triggered from other channels (e.g., asking
nanobot on Feishu to send an email).

Now `autoReplyEnabled` only controls automatic replies to incoming
emails, not proactive sends. This allows users to disable auto-replies
while still being able to ask nanobot to send emails on demand.

Changes:
- Check if recipient is in `_last_subject_by_chat` to determine if
  it's a reply
- Only skip sending when it's a reply AND auto_reply_enabled is false
- Add test for proactive send with auto_reply_enabled=false
- Update existing test to verify reply behavior

nanobot/channels/email.py

@@ -108,11 +108,6 @@ class EmailChannel(BaseChannel):
             logger.warning("Skip email send: consent_granted is false")
             return
 
-        force_send = bool((msg.metadata or {}).get("force_send"))
-        if not self.config.auto_reply_enabled and not force_send:
-            logger.info("Skip automatic email reply: auto_reply_enabled is false")
-            return
-
         if not self.config.smtp_host:
             logger.warning("Email channel SMTP host not configured")
             return
@@ -122,6 +117,15 @@ class EmailChannel(BaseChannel):
             logger.warning("Email channel missing recipient address")
             return
 
+        # Determine if this is a reply (recipient has sent us an email before)
+        is_reply = to_addr in self._last_subject_by_chat
+        force_send = bool((msg.metadata or {}).get("force_send"))
+
+        # autoReplyEnabled only controls automatic replies, not proactive sends
+        if is_reply and not self.config.auto_reply_enabled and not force_send:
+            logger.info("Skip automatic email reply to {}: auto_reply_enabled is false", to_addr)
+            return
+
         base_subject = self._last_subject_by_chat.get(to_addr, "nanobot reply")
         subject = self._reply_subject(base_subject)
         if msg.metadata and isinstance(msg.metadata.get("subject"), str):