Add RunSubagentOptions.onEvent callback so callers (TUI live update panel
for /delegate, /rubber-duck, etc.) get every session event without polling.
Errors from the callback are caught so a buggy caller cannot crash the agent.
Chain caller-supplied AbortSignal through a local AbortController in
runSingleAgent and register it in a new liveSubagentControllers set so
stopLiveSubagents aborts in-process subagents alongside the legacy spawn-based
processes (cmux split, sift codebase_search).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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