fix(core): move SIGHUP handler into exit context for proper cleanup

This restores the SIGHUP handler that was previously reverted. The handler ensures the process exits on terminal hangup (eg. closing the terminal tab), preventing long-running commands like `serve` from becoming orphaned processes.

packages/opencode/src/cli/cmd/tui/context/exit.tsx

@@ -53,6 +53,7 @@ export const { use: useExit, provider: ExitProvider } = createSimpleContext({
         message: store,
       },
     )
+    process.on("SIGHUP", () => exit())
     return exit
   },
 })