Bug Description

When a sub-agent is spawned via sessions_spawn from a channel session (e.g. Discord), the completion announcement (containing task summary, token stats, session key, transcript path) is delivered to the channel session as a regular message. The LLM in that session then posts it to the channel — leaking internal processing details to external users.

Reproduction

1. Configure a Discord channel with a system prompt that spawns sub-agents for incoming bot messages
2. Have another bot send a message to the channel
3. The channel session spawns a sub-agent via sessions_spawn
4. When the sub-agent completes, the completion report appears as a message in the channel session
5. The LLM posts it to Discord, exposing:
   • Internal reasoning chain
   • Token usage stats
   • Session keys and transcript paths
   • What was shared vs withheld

Expected Behavior

Sub-agent completion announcements should not route to the originating channel session. They should either:

1. Route only to the parent session (main session that spawned the channel session), or
2. Be suppressed entirely when sessions_spawn is called from a channel session, or
3. Be tagged as [internal] so the channel session can reliably detect and suppress them

Current Workaround

Added a system prompt rule: "When you receive a [System Message] about a subagent completing, reply with ONLY: NO_REPLY". This works ~80% of the time but is not reliable — the LLM sometimes still posts the completion report.

Impact

• Security: Exposes internal processing chain to external/untrusted users (other bots, guest humans)
• Privacy: Leaks token stats, session keys, transcript paths
• Trust: In an A2A (agent-to-agent) context, the visiting agent sees exactly what was shared and what was withheld, undermining the sandboxing model

Environment

• OpenClaw version: latest
• Channel: Discord
• Context: Agent-to-agent sandbox with sub-agent isolation

Suggested Fix

The gateway should filter completion announcements at the routing layer, before they reach the channel session's LLM. A channel session spawning a sub-agent should never see the raw completion report in its message stream.