feat: add graceful SIGTERM handling for clean shutdown

Install a SIGTERM signal handler that stops the bot loop and closes
the IRC connection, allowing cProfile stats to flush before exit.

src/derp/cli.py

@@ -49,13 +49,27 @@ def build_parser() -> argparse.ArgumentParser:
 
 
 def _run(bot: Bot) -> None:
-    """Run the bot event loop."""
+    """Run the bot event loop with graceful SIGTERM handling."""
+    import signal
+
+    async def _start_with_signal():
+        loop = asyncio.get_running_loop()
+        loop.add_signal_handler(signal.SIGTERM, _shutdown, bot)
+        await bot.start()
+
     try:
-        asyncio.run(bot.start())
+        asyncio.run(_start_with_signal())
     except KeyboardInterrupt:
         logging.getLogger("derp").info("interrupted, shutting down")
 
 
+def _shutdown(bot: Bot) -> None:
+    """Signal handler: stop the bot loop so cProfile can flush."""
+    logging.getLogger("derp").info("SIGTERM received, shutting down")
+    bot._running = False
+    asyncio.get_running_loop().create_task(bot.conn.close())
+
+
 def main(argv: list[str] | None = None) -> int:
     """Main entry point."""
     parser = build_parser()