docs: document sub-agent event attribution

[loganrosen]

Summary

• Document envelope-level agentId in the streaming events guide
• Add parent-agent-only rendering guidance for sub-agent event streams
• Mark payload-level parentToolCallId as deprecated for sub-agent attribution
• Note shared sub-agent event streams in custom agents docs

Validation

• just validate-docs-ts
• Syntax-checked changed streaming-events.md Python snippets with python3 -m py_compile
• Documentation validation is passing in CI

[Copilot]

Pull request overview

Updates the documentation to clarify how sub-agent attribution works in streamed session events, centering on the envelope-level agentId and guidance for rendering only the parent agent’s chat output while routing sub-agent activity elsewhere.

Changes:

• Documented envelope-level agentId in the streaming event envelope and added renderer guidance to ignore sub-agent assistant events in the main chat UI.
• Marked payload-level parentToolCallId as deprecated for sub-agent attribution across relevant event payload tables.
• Noted in custom agents docs that sub-agent events share the parent session stream and can be separated via the envelope.

Show a summary per file

File	Description
docs/features/streaming-events.md	Adds agentId to the event envelope docs, introduces “render only the parent agent” guidance, and deprecates parentToolCallId for attribution.
docs/features/custom-agents.md	Adds a clarification that sub-agent events share the parent stream and can be separated using envelope-level agentId.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 1
• Review effort level: Low