feat(tui): add audio notification system for completion and approval events

[echobt]

Summary

Add audio notification system to the TUI that plays sounds when:

• Agent response completes streaming
• Tool/plan approval is requested

Changes

• Add rodio dependency for cross-platform audio playback
• Create sound.rs module with channel-based audio thread architecture
• Embed WAV files for response completion and approval notification sounds
• Play completion sound when streaming response finishes
• Play approval sound when tool approval is requested
• Graceful fallback to terminal bell when audio unavailable
• Add sound toggle in settings menu

Technical Details

• Uses bounded channel (capacity 16) to prevent memory exhaustion
• Named thread (cortex-audio) for debugging
• Non-blocking try_send to avoid UI freezes
• Flushes stdout when falling back to terminal bell

Testing

• Code compiles successfully with cargo check -p cortex-tui
• Unit tests added for sound module