[Feature Request]Audio notification on task completion

Description:
Add an optional audio notification when Claude Code returns to
prompt-waiting state after completing a task.

Use case:
Users working on long-running tasks would benefit from audio feedback
when Claude finishes, especially when multitasking.

Suggested implementation:
```yaml
# ~/.claude/config.yml
notifications:
on_startup:
command: "say 'Claude Code ready'"
enabled: true

on_idle: # After completing task, waiting for input
command: "say 'Task completed'"
enabled: true

on_exit:
command: "say 'Goodbye'"
enabled: false

on_error: # When task fails
command: "say 'Error occurred'"
enabled: false

# Custom events
on_git_commit:
command: "say 'Code committed'"
enabled: false

Cross-platform commands:

• macOS: say "Task completed"
• Linux: espeak "Task completed" or spd-say "Task completed"
• Windows: powershell -c "Add-Type -AssemblyName System.Speech;

(New-Object System.Speech.Synthesis.SpeechSynthesizer).Speak('Task
completed')"

• Fallback: System beep (echo -e "\a")

Benefits:

• No external dependencies (uses system commands)
• Optional/configurable
• Cross-platform support
• Improves user experience for long tasks