fix(review,investigate): kill agent process groups so Ctrl+C can't hang

Agent CLIs (codex, claude, ...) spawn helper grandchildren (sandbox, MCP servers) that inherit the agent's stdout pipe. exec.CommandContext's default Cancel only SIGKILLs the agent itself, so on Ctrl+C / reviewer timeout the grandchildren stay alive holding the pipe open. The reviewer template drains Events() (the stdout pipe) to EOF before calling Wait, so the read blocks forever, the per-agent goroutine never finishes, the fan-in channel never closes, and RunMulti hangs — the dashboard sits on "cancelling" indefinitely. Investigate's cmd.Run has the same exposure.

Add procutil.TerminateOnCancel (process-group SIGKILL on cancel + a WaitDelay backstop) and apply it on the agent spawn path in both the review template and the investigate loop. Mirrors the existing checkpoint/remote git-transport fix.