Subagents link to the parent's abort and report runId and usage-limit denials

What

A subagent is a helper copy of Claude that your session starts to handle part of a task, either while you wait (synchronous) or in the background. Several things about how they start and report back changed.

• Background subagents: the abort controller (what stops a run when you cancel) is now linked to the parent's abort controller through a relay that can be detached.
• Synchronous subagents: launches now pass parentAbortController and get back detachParentAbortRelay, which is called when the subagent finishes. Before, a standalone relay was used.
• runId: task notifications from subagents running on their own now include it.
• usageLimitDenial: passed along when a run fails because of a usage limit.
• launchedWithDefaults: recorded for local and background tasks.
• Remote subagent launches now work out their permission mode through a new helper that takes declaredMode, parentModeToHand and model.

Why

Cancelling should now reach subagents more reliably, and the main session or an interface showing subagents can tell a usage-limit failure apart from other failures.