fix(messages): normalize webhook reply references

AGENTS.md

@@ -28,11 +28,31 @@ deploy/       # Deployment artifacts
   quickstart/ # One-shot local/UAT Compose stack (PG+pgvector+Redis+Meilisearch+Mailhog)
   fluentd/    # Log shipping config
 docs/         # Project documentation (architecture, requirements, plans, reports)
+  chatwoot/   # Upstream Chatwoot source snapshot; primary behavioral reference
 ```
 
 The **Go module root is `backend/`** — run all `go` commands from there.
 Build context for Docker is the **repository root** (`.`), not `backend/`.
 
+## Upstream Chatwoot Reference (Required)
+
+GoChat is a Go-language translation and port of Chatwoot. When a behavior,
+API contract, data model, event payload, frontend interaction, or edge case is
+unclear or appears incorrect, **inspect the upstream implementation under
+`docs/chatwoot/` before designing or changing the GoChat solution**.
+
+- Trace the complete upstream path first: controller/handler → service/builder
+  → model callback → serializer → frontend consumer.
+- Preserve Chatwoot's externally visible behavior and payload contracts when
+  translating Rails/Ruby implementation details into GoChat's layered Go
+  architecture.
+- Reuse the upstream fix pattern and regression cases where practical instead
+  of inventing a parallel behavior from the current Go code alone.
+- Treat `docs/chatwoot/` as read-only reference code. Production changes belong
+  in `backend/`, `frontend/`, or other GoChat-owned directories.
+- If GoChat intentionally diverges from Chatwoot, document the reason, affected
+  contract, and verification coverage in the change.
+
 ## Build & Test Commands
 
 All Go commands run from `backend/`: