Add Spotify integration: control local Spotify playback volume

[joshuamckenty]

Instead of playing bundled audio files, codevator should optionally control the volume of the user's local Spotify web player output.

Behavior:

• When Claude starts working (PreToolUse hook), fade Spotify volume up
• When Claude stops/goes idle, fade Spotify volume down
• User's existing Spotify playlist continues playing — codevator just manages the volume

Implementation ideas:

• Use Spotify Web API or AppleScript (osascript) to control Spotify.app volume on macOS
• Could be a new mode: codevator mode spotify
• Needs Spotify OAuth or local AppleScript (no auth needed for local control)

Prior art:

• macOS: osascript -e 'tell application "Spotify" to set sound volume to 50'
• Spotify Web API: PUT /v1/me/player/volume?volume_percent=50