watch: outbound messages echoed as is_from_me=false duplicates

[egantz]

Summary

When sending a message via imsg send, the watch stream emits two records for the same message:

1. is_from_me: true (correct outbound)
2. is_from_me: false with identical text (spurious duplicate)

Evidence

{"id":753031,"is_from_me":true,"text":"Test message...","guid":"896ECCAD-..."}
{"id":753032,"is_from_me":false,"text":"Test message...","guid":"75BDDF1E-..."}

Same text, different GUIDs, different ids, opposite is_from_me values.

Impact

Clients using watch to build chat bots filter is_from_me: true to avoid echo loops, but the is_from_me: false duplicate still triggers inbound handling.

Suggested fix

Deduplicate by content hash + timestamp within a short window (e.g., 2s), or investigate why macOS Messages stores these delivery-confirmation records.

Environment

• imsg 0.5.0
• macOS 25.4 (arm64)
• Clawdbot gateway consuming watch stream