fix: prevent self-kill and ensure graceful shutdown with terminal flush - #886

I'll have my agents take a look

Thanks for digging into the exit-cleanup path — the underlying idea (make sure buffered terminal escape sequences reach the tty before the process dies) is legitimate and worth pursuing. A few concerns before this is portable, though:

1. command-registry.ts: replacing process.kill(process.pid, 'SIGINT') with process.exit(0) changes behavior, not just timing. The SIGINT handler (via use-exit-handler) presumably drives the async cleanup path in freebuff-exit.ts (endFreebuffSessionBestEffort, flushAnalytics, stopActiveRun). process.exit(0) only fires the synchronous 'exit' listeners, so the exit/quit/q command will now skip session-end and analytics flushing entirely. That's a regression, not a fix, unless those calls are also wired into the 'exit' handler.

2. renderer-cleanup.ts: process.stdout._handle?.setBlocking?.(true) reaches into a private, untyped Node internal (_handle isn't on the WriteStream type), which will likely fail TypeScript compilation without an any cast and is fragile across Node/Bun versions. Also, reusing handlersInstalled (previously an installation guard) to also guard against double-cleanup() calls conflates two unrelated concerns and could silently disable handler re-installation elsewhere.

3. run-terminal-command.ts: the child.pid === process.pid guards in killProcessGroup/exit sweep protect against a scenario (pid collision between parent and its own child) that essentially can't happen under normal OS pid allocation. This reads as defensive code added without a concrete repro, which raises the question of whether the actual root cause of the terminal garbling was diagnosed.

The freebuff-exit.ts flush addition is the most clearly beneficial and low-risk piece here. I'd suggest splitting that out, dropping the self-kill guards unless you have a reproducible case, and reverting the exit command to keep the async cleanup path intact (or explicitly wiring cleanup into process.on('exit') if you want the synchronous route).