[Bug] System-level "Exec completed/failed" notifications leak into unrelated webchat sessions

Summary

System-generated notifications for background exec completions ("Exec completed (session-id, code N)", "Exec failed (session, signal SIGTERM)") are appearing in webchat sessions that did not initiate the exec. This causes confusion as users see completion messages for tasks they didn't start.

This appears related to #25959 (which was closed as stale), but the bug persists in the latest release.

Environment

• OpenClaw version: 2026.4.12 (latest as of report date)
• OS: Ubuntu 25.10
• Session type: Control UI (webchat) via Tailscale
• Channel: webchat

Steps to Reproduce

1. User is in a webchat session (Control UI)
2. Some other process (subagent, cron job, or background exec) runs and completes
3. System message appears in webchat: [timestamp] Exec completed (session-name, code 0)
4. User sees notification for work they did not initiate

Observed Behavior

System events from other sessions are being delivered to the webchat session:

[2026-04-14 10:43:18 CDT] Exec failed (gentle-s, signal SIGTERM)
[2026-04-14 10:43:18 CDT] Exec failed (marine-c, signal SIGTERM)
[2026-04-14 10:43:53 CDT] Exec completed (keen-slu, code 0)

These sessions (gentle-s, marine-c, keen-slu) were not started by the webchat user.

Expected Behavior

• System-level exec notifications should only appear in the session that initiated the exec
• OR there should be a configuration option to suppress system-level async notifications entirely
• Webchat sessions should not receive completion events from unrelated background work

Impact

• User confusion: "I thought this was stopped now" — user sees messages they don't understand
• Context pollution: System messages interleave with user conversation
• Wasted tokens: System events consume context window

Additional Context

• Issue [Bug] "Exec completed" system message causes duplicate user message processing #25959 reported similar symptoms but was closed as stale
• User explicitly requested suppression of these notifications: "I want it stopped. Causes tons of confusion."
• This is NOT about inline vs background exec (which is already configured correctly)
• This IS about system-level event routing/delivery across sessions

Related Issues

• [Bug] "Exec completed" system message causes duplicate user message processing #25959 - [Bug] "Exec completed" system message causes duplicate user message processing
• [Bug]: async task completion reports can be lost because system event/wake is not reliably session-targeted #52305 - async task completion reports can be lost because system event/wake is not reliably session-targeted
• [Bug]: Telegram topic + busy queue + heartbeat/system events can replay turns and break voice-note handling #58418 - Telegram topic + busy queue + heartbeat/system events can replay turns

Request

Either:

1. Fix session targeting so exec notifications only go to originating sessions
2. Provide a config option to suppress all system-level async completion notifications
3. Document the workaround if one exists