Allow cancelling running task processes

[beNative]

Summary

• track spawned task processes in the main process and add an IPC handler to terminate them cleanly when requested
• extend the renderer repository manager to request cancellations and update the UI with cancel controls on repository cards and the task log panel
• propagate cancellation-aware events through the preload bridge and types so cancelled steps are surfaced without being treated as failures

Testing

• npm test

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https://chatgpt.com/codex/tasks/task_e_68f4fb6176a08332862a8a3b75ffc739