fix(signal): handle Note to Self messages with echo-back protection

Support Signal 'Note to Self' messages in single-number setups where
signal-cli is linked as a secondary device on the user's own account.

syncMessage.sentMessage envelopes addressed to the bot's own account
are now promoted to dataMessage for normal processing, while other
sync events (read receipts, typing, etc.) are still filtered.

Echo-back prevention mirrors the WhatsApp bridge pattern:
- Track timestamps of recently sent messages (bounded set of 50)
- When a Note to Self sync arrives, check if its timestamp matches
  a recent outbound — skip if so (agent echo-back)
- Only process sync messages that are genuinely user-initiated

Based on PR #2115 by @Stonelinks with added echo-back protection.

gateway/platforms/signal.py

@@ -179,6 +179,11 @@ class SignalAdapter(BasePlatformAdapter):
         # Normalize account for self-message filtering
         self._account_normalized = self.account.strip()
 
+        # Track recently sent message timestamps to prevent echo-back loops
+        # in Note to Self / self-chat mode (mirrors WhatsApp recentlySentIds)
+        self._recent_sent_timestamps: set = set()
+        self._max_recent_timestamps = 50
+
         logger.info("Signal adapter initialized: url=%s account=%s groups=%s",
                      self.http_url, _redact_phone(self.account),
                      "enabled" if self.group_allow_from else "disabled")
@@ -353,10 +358,26 @@ class SignalAdapter(BasePlatformAdapter):
         # Unwrap nested envelope if present
         envelope_data = envelope.get("envelope", envelope)
 
-        # Filter syncMessage envelopes (sent transcripts, read receipts, etc.)
-        # signal-cli may set syncMessage to null vs omitting it, so check key existence
+        # Handle syncMessage: extract "Note to Self" messages (sent to own account)
+        # while still filtering other sync events (read receipts, typing, etc.)
+        is_note_to_self = False
         if "syncMessage" in envelope_data:
-            return
+            sync_msg = envelope_data.get("syncMessage")
+            if sync_msg and isinstance(sync_msg, dict):
+                sent_msg = sync_msg.get("sentMessage")
+                if sent_msg and isinstance(sent_msg, dict):
+                    dest = sent_msg.get("destinationNumber") or sent_msg.get("destination")
+                    sent_ts = sent_msg.get("timestamp")
+                    if dest == self._account_normalized:
+                        # Check if this is an echo of our own outbound reply
+                        if sent_ts and sent_ts in self._recent_sent_timestamps:
+                            self._recent_sent_timestamps.discard(sent_ts)
+                            return
+                        # Genuine user Note to Self — promote to dataMessage
+                        is_note_to_self = True
+                        envelope_data = {**envelope_data, "dataMessage": sent_msg}
+            if not is_note_to_self:
+                return
 
         # Extract sender info
         sender = (
@@ -371,8 +392,8 @@ class SignalAdapter(BasePlatformAdapter):
             logger.debug("Signal: ignoring envelope with no sender")
             return
 
-        # Self-message filtering — prevent reply loops
-        if self._account_normalized and sender == self._account_normalized:
+        # Self-message filtering — prevent reply loops (but allow Note to Self)
+        if self._account_normalized and sender == self._account_normalized and not is_note_to_self:
             return
 
         # Filter stories
@@ -577,9 +598,18 @@ class SignalAdapter(BasePlatformAdapter):
         result = await self._rpc("send", params)
 
         if result is not None:
+            self._track_sent_timestamp(result)
             return SendResult(success=True)
         return SendResult(success=False, error="RPC send failed")
 
+    def _track_sent_timestamp(self, rpc_result) -> None:
+        """Record outbound message timestamp for echo-back filtering."""
+        ts = rpc_result.get("timestamp") if isinstance(rpc_result, dict) else None
+        if ts:
+            self._recent_sent_timestamps.add(ts)
+            if len(self._recent_sent_timestamps) > self._max_recent_timestamps:
+                self._recent_sent_timestamps.pop()
+
     async def send_typing(self, chat_id: str, metadata=None) -> None:
         """Send a typing indicator."""
         params: Dict[str, Any] = {
@@ -635,6 +665,7 @@ class SignalAdapter(BasePlatformAdapter):
 
         result = await self._rpc("send", params)
         if result is not None:
+            self._track_sent_timestamp(result)
             return SendResult(success=True)
         return SendResult(success=False, error="RPC send with attachment failed")
 
@@ -665,6 +696,7 @@ class SignalAdapter(BasePlatformAdapter):
 
         result = await self._rpc("send", params)
         if result is not None:
+            self._track_sent_timestamp(result)
             return SendResult(success=True)
         return SendResult(success=False, error="RPC send document failed")