Add Ghostty split-pane backend via AppleScript (macOS)

Feature Request

Add a GhosttyBackend for agent team split panes on macOS, using Ghostty's AppleScript API.

Context

The ITermBackend was recently fixed (v2.1.77) and works great for iTerm2 users. Ghostty is a popular modern terminal on macOS that supports a rich AppleScript object model for programmatic split pane control — everything needed to implement a backend equivalent to ITermBackend.

Ghostty's AppleScript API

Ghostty exposes the following relevant AppleScript capabilities:

tell application "Ghostty"
    -- Create a configuration with working directory and command
    set cfg to new surface configuration
    set initial working directory of cfg to "/path/to/dir"
    set command of cfg to "/bin/zsh"

    -- Split a pane (directions: right, left, down, up)
    set pane2 to split pane1 direction right with configuration cfg

    -- Send text/keys to a specific pane
    input text "echo hello" to pane2
    send key "enter" to pane2

    -- Query pane properties
    get id of pane2
    get working directory of pane2

    -- Close a pane
    close pane2

    -- Other actions
    perform action "equalize_splits" on pane1
end tell

This maps directly to what ITermBackend does with the it2 CLI:

| ITermBackend (it2 CLI) | GhosttyBackend (AppleScript) |
|---|---|
| it2 session split -v | split terminal direction right with configuration cfg |
| it2 session split -s <id> | split terminal direction down with configuration cfg |
| Send command via CLI | input text "cmd" to terminal + send key "enter" |
| Parse session ID from output | get id of terminal |
| Kill pane | close terminal |

Implementation Notes

• AppleScript calls can be made via osascript -e '...' (similar to how ITermBackend shells out to it2)
• Ghostty detection: check TERM_PROGRAM=ghostty env var
• AppleScript is enabled by default in Ghostty (macos-applescript = true)
• macOS TCC automation permissions may prompt the user on first use
• teammateMode: "ghostty" and "auto" mode detection would both be needed (like iTerm2)

Environment

• Platform: macOS
• Terminal: Ghostty
• Claude Code: v2.1.77