fix(agents): sound scripts exit silently on error conditions

[homich1991]

Summary

Updates the Claude plugin sound scripts (bash and PowerShell) to exit
silently (with code 0) instead of displaying error messages when optional
conditions fail. This prevents unwanted error output in the CLI when
sound functionality is not available or configured.

Changes

• Modified both play-random-sound.sh and play-random-sound.ps1 to
  exit with code 0 instead of code 1 for all error conditions
• Directory not found: exits silently instead of showing error message
• No audio files found: exits silently instead of showing error message
• No audio player found: exits silently instead of showing error message
• Updated comments to clarify silent exit behavior

Impact

Before: When sound directory doesn't exist or audio player is not
installed, users see error messages in CLI output even though sounds are
optional features.

After: Scripts exit gracefully without any error output, providing
cleaner CLI experience when sounds are not configured.

Checklist

• Self-reviewed
• Manual testing performed
• Documentation updated (if needed)
• No breaking changes (or clearly documented)