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.
ec9b868aea