Scope completion notifications to direct children

### Describe the feature or problem you'd like to solve

Orchestrating agents receive completion noise from their entire descendant tree, not just direct children

### Proposed solution

Completions should propagate one level: each agent receives signals only from its direct children. An orchestrator launching 8 sub-agents gets 8 completion signals regardless of how much nested work each spawns — keeping it focused on its own layer and reducing context noise from work it didn't directly delegate.

### Example prompts or workflows

1. An orchestrator runs 8 sub-agents, each using explore agents internally — it receives ~40–50 notifications today when 8 is all it needs.
2. A sub-agent runs shell commands; each completion fires a notification to the orchestrator even though it has no use for them.
3. Orchestrator → coordinators → workers: the orchestrator should hear from coordinators only, not from workers two levels down.

### Additional context

_No response_

No files, tests, or entry points are identified in the issue. Start by locating how completion notifications propagate through nested agents, then trace the parent-child relationship used for delivery. Done means each agent receives completion signals only from its direct children, with nested descendants no longer producing extra notifications.

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