Sub-agent inter-session messages duplicated multiple times

Description

When using sessions_send to communicate with persistent sub-agents, the sub-agent's reply gets delivered multiple times (2-3 times) to the parent session's conversation. This causes severe clutter, especially in webchat UI.

Steps to Reproduce

1. Create a persistent sub-agent session
2. From the main session, call sessions_send to the sub-agent with a simple message
3. Observe the conversation: the sub-agent's reply appears 2-3 times as inter-session messages

Expected Behavior

Sub-agent replies should appear only once in the parent session's conversation flow.

Actual Behavior

The same reply appears 2-3 times. Additionally, the delivery.mode: "announce" on sub-agent sessions causes the reply to be pushed to channels, which may trigger another round of inter-session messages if the sub-agent responds to the announce notification (creating a feedback loop).

Environment

• OpenClaw: 2026.4.24
• Sub-agent type: persistent session via sessions_spawn (runtime=subagent, mode=session)
• streamTo: "parent" configured

Possible Causes

1. Inter-session message delivery has retry/redundancy logic that fires multiple times
2. delivery.mode: "announce" on sub-agent sessions creates a secondary delivery path that overlaps with the sessions_send reply path
3. Sub-agent receiving its own announce notification and responding creates a feedback loop

Suggested Fix

1. Deduplicate inter-session messages before injecting into conversation
2. Allow disabling delivery.mode: "announce" for sub-agent sessions (or default to "none")
3. Prevent sub-agents from receiving their own announce notifications as inter-session messages