fix(orchestrator): Keep Claude child work on the child thread

[mwolson]

What Changed

A Claude child can keep talking after the parent root settles. Later text, tools, and results were landing on the parent thread, so the child's work showed up in the wrong conversation. Those later frames now stay on the child.

A blank or whitespace Agent prompt, or progress that arrived before the real prompt, opened the child with an empty "Sent by another agent" bubble. The child now waits for real prompt text before showing that opening message. A child that never gets a prompt still runs. It just starts with no blank bubble.

Checked on a live Claude thread. One Agent launched with a single space, another with a real prompt. The blank child had no user message. The sibling had exactly one opening message with the real prompt.

Why

Settle on the parent is not the end of that child's stream. Routing later frames with the parent's turn mixed the two conversations.

The empty bubble was the same class of mistake: the child thread showed a user message that nobody sent.

Review both here. #5388 claude-postsettle-attribution is closed as absorbed.

UI Changes

Child output after the parent settles stays in the child thread on web and mobile. A blank Agent launch no longer creates an empty "Sent by another agent" bubble.

No before/after screenshots.

Checklist

• This PR is small and focused
• I explained what changed and why
• I included before/after screenshots for any UI changes
• I included a video for animation/interaction changes

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Note

Low Risk
Narrow behavioral change in ACP subagent UI projection with no auth, persistence, or routing logic touched.

Overview
When an ACP provider registers a subagent child thread, the adapter only projects the opening user message and turn item if hasSubagentPromptText(update.prompt) is true (non-empty after trim).

Previously, every subagent launch always emitted message.updated and turn_item.updated for the prompt, which could surface an empty “Sent by another agent” bubble when the agent sent a whitespace-only prompt or progress arrived before real prompt text.

Child thread creation and subagent metadata emission are unchanged; only the optional prompt conversation artifacts are gated.

^{Reviewed by Cursor Bugbot for commit ed223fe. Bugbot is set up for automated code reviews on this repo. Configure here.}

Note

Fix Claude child thread output attribution after root thread settlement

• Adds hasSubagentPromptText to suppress empty opening user message bubbles in child threads when a subagent prompt is blank, whitespace-only, null, or undefined; applied across Claude, ACP, Codex, and Cursor adapters.
• Reworks session-scoped subagent state in ClaudeAdapterV2 to track subagent lineages per native thread across turns, serializing SDK message handling via a semaphore and using generation-based guards to prevent duplicate emissions.
• Adds durable alias registration so post-settlement tool results and assistant text with parent_tool_use_id are routed to the correct child thread rather than leaking to the root.
• Buffers unresolved subagent messages until their lineage alias registers, with overflow protection (>32 pending launches disables implicit alias recovery).
• Hides provider-buffered automatic continuation messages (text 'Background task completed.') from the visible run queue in deriveThreadQueueWorkflowState.
• Risk: Large refactor of ClaudeAdapterV2 internal state; subagent indexing moved from per-turn to session scope, which changes how interrupts, session closes, and failed wake drains are handled.

^{Macroscope summarized ed223fe.}

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