fix(tracing): wire flushSync into the TUI worker exit/SIGINT path (M3 crash-guard never fires in pure-TUI) #875

Found during v0.8.1 release review (CTO, P2). The M3 crash-guard added in #865 only runs when flushSync fires, which is wired in run.ts (CLI SIGINT/exit) but not in worker.ts. In pure-TUI sessions the M3 export-vs-flushSync path is never exercised. Not a regression (no worse than before #865); add a flushSync call in a TUI exit/SIGINT handler so the guard actually protects TUI traces.