[Feature Request] Add windowTitle config option for customizing terminal window title

Bug Description

Feature request: windowTitle config option

Similar to how statusLine allows a custom command to set the bottom status bar, I'd like a windowTitle option in settings.json to customize the terminal window title.

Use case: I run multiple Claude Code sessions in different Ghostty tabs/windows and can't tell them apart. The current title is set automatically but I can't customize it.

Proposed behavior:

{
  "windowTitle": {
    "type": "command",
    "command": "echo \"$SESSION_NAME - $(basename $PWD)\""
  }
}

Or simpler — just expose $CLAUDE_SESSION_NAME as an env var so users can set titles via hooks or shell config.

Current workaround: Custom /set-title command, but this requires manual invocation and doesn't have access to session name.

Environment Info

• Platform: darwin
• Terminal: ghostty
• Version: 2.1.15
• Feedback ID: 3857bf43-62cf-4377-8503-af30f5afcaa6