fix(muse): align background roster task IDs with subagent turn items #23844

Summary

Align the Muse provider-thread background roster task ID with the native ID used by the corresponding subagent turn item.

Requested by @koushikxd as a follow-up to PR #23828.

Problem

In packages/provider-muse/src/server/adapter.ts, syncBackground lists a running child on the provider-thread roster under itemId. The corresponding turn item's nativeItemRef uses childSessionId ?? subagentId ?? itemId.

When these IDs differ:

• The web banner cannot filter out a follow-up turn's own children by matching their turn-item IDs.
• derivePendingBackgroundWork cannot deduplicate the roster entry against the corresponding turn item.

This can display the same running child in both the timeline and the background-work banner.

Required changes

• Inspect Muse roster creation, stop handling, and roster cleanup paths that match on taskId.
• Use the same native ID selection for the roster entry and its corresponding turn item, likely childSessionId ?? subagentId ?? itemId.
• Update dependent stop and cleanup matching as needed so that child completion, failure, or cancellation removes the correct roster entry.
• Add regression coverage for differing itemId, childSessionId, and subagentId values, including fallback behavior.

Affected areas

• packages/provider-muse/src/server/adapter.ts: syncBackground, turn-item native references, and related stop/cleanup handling.
• packages/shared/src/orchestrationV2PendingBackgroundWork.ts: identity-based deduplication behavior.
• apps/web/src/components/ChatView.logic.ts: active-turn roster filtering.
• Relevant Muse adapter and background-work derivation tests.

Acceptance criteria

• A Muse child uses the same identity in the provider-thread roster and its corresponding turn item's native reference.
• A child started by an active follow-up turn remains in the timeline and does not also appear in the background-work banner.
• An earlier turn's pending child remains visible in the banner while the follow-up turn runs.
• Shared background-work derivation does not return duplicate entries for the same child.
• Stop and roster cleanup continue to target the correct child when native IDs differ from itemId.
• Tests cover native-ID precedence and the itemId fallback.

Out of scope

The Claude background-command case is excluded. Its tool item completes when the command is backgrounded, so the banner entry is not a duplicate of a running tool item.

References

• PR: fix(web): subagent banner stays visible while a follow-up turn runs #23828
• Review discussion and follow-up request: fix(web): subagent banner stays visible while a follow-up turn runs #23828 (comment)