Registers the Google Antigravity IDE so the workspace Open-in-editor
picker detects and launches it (PATH probe for the "antigravity"
command), alongside Cursor, VS Code, WebStorm, and Zed.
- desktop: add the target to BUILT_IN_EDITOR_TARGETS (kind: editor).
- app: add "antigravity" to the known editor-target id set and map its
bundled icon.
Co-authored-by: Mathias Kurz <mkurz@stamus-networks.com>
