security: Audit external process execution

## What
Security audit for osascript command execution in system_audio/mod.rs.

## Why
- External process execution via osascript for Homebrew installation (lines 201-210)
- Potential command injection risk if user input is introduced
- Currently arguments are hardcoded but future changes may introduce risk

## How
- Audit all command execution points
- Ensure no user input flows into command arguments
- Document security considerations
- Consider using native AppleScript bindings instead of osascript