whatsapp: prepend sender identity to group messages

Bridge extracts senderId (participant JID) + senderName (pushName) on
every inbound message and posts them in the HTTP event dict, but the
gateway was dropping both: `text = body` carried only the plain message
content. That meant all group members' messages arrived at the LLM
indistinguishable from the session owner's, so (a) the LLM couldn't
attribute a question to the actual asker, (b) memory-write tool calls
triggered by a guest were stored under the owner's profile, and (c)
the trust-tier policy in AGENTS.md was effectively unenforceable.

Prepend `[<pushName>] ` to body for `isGroup` events only, after
@bot-mention stripping + doc-content injection so the prefix stays at
the very start of the text the LLM reads. Fallback order:
pushName → short JID → "unknown". DMs untouched (no ambiguity).

Completes the design intent already documented in
gateway/session.py:245-256 ("individual sender names are prefixed on
each user message") — that comment promised behavior that was never
implemented in code.

gateway/platforms/whatsapp.py

@@ -1060,6 +1060,20 @@ class WhatsAppAdapter(BasePlatformAdapter):
                         except Exception as e:
                             print(f"[{self.name}] Failed to read document text: {e}", flush=True)
 
+            # Prepend sender identity on group messages so the LLM can
+            # attribute each turn to the correct participant. Without this,
+            # every group member's writes arrive indistinguishable from the
+            # session owner's, which breaks memory-write attribution and
+            # defeats the trust-tier policy documented in AGENTS.md. DMs are
+            # untouched since there's only one possible sender. Fallback
+            # order: pushName → short JID → "unknown".
+            if is_group and body:
+                sender_name = data.get("senderName")
+                if not sender_name:
+                    sid = data.get("senderId", "") or ""
+                    sender_name = sid.split("@")[0] if sid else "unknown"
+                body = f"[{sender_name}] {body}"
+
             return MessageEvent(
                 text=body,
                 message_type=msg_type,