Ghostty: align launchers with Raycast best practices

[rolldav]

Description

This PR aligns the Ghostty AI terminal launchers with Raycast script-command best practices.

Changes included:

• Use the Ghostty binary resolved by find_ghostty instead of hard-depending on Ghostty.app.
• Deduplicate Ghostty discovery logic into a shared helper (lib/ghostty.sh) and source it from all launchers.
• Remove unnecessary disown calls (Raycast scripts are non-interactive; background & is sufficient).
• Restore the missing README screenshot.
• Make all script output and documentation fully English.
• Remove the untraceable Validated: comment.

Type of change

• [ ] New script command
• [x] Improvement of an existing script
• [ ] Bug fix
• [x] Documentation update
• [ ] Toolkit change
• [ ] Other

Screenshot

Dependencies / Requirements

• Ghostty terminal must be installed and available in PATH (e.g. brew install ghostty)