Feature: Notification when background Bash command completes

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

When using Bash tool with run_in_background: true, there's no notification when the command finishes. Users have to manually check with TaskOutput or read the output file to know if it's done.

This is especially painful for long-running commands like:

• pnpm build
• pnpm test
• Large file operations

Proposed Solution

Add a desktop/terminal notification when a background Bash command completes, similar to how terminal apps like iTerm2 notify when a long command finishes.

Options:

1. macOS native notification via osascript
2. Terminal bell (\a)
3. A visible indicator in the Claude Code TUI

Additional Context

Related to #20525 (background Task completion notification), but this is specifically for the Bash tool's run_in_background parameter.

Current workaround is polling with TaskOutput(task_id, block=false), which is tedious.