Add Google Antigravity installer and integrate into Ubuntu/Omarchy install

[kalashnikovisme]

Motivation

• Provide a small desktop launcher to open Google Antigravity (https://elgoog.im/gravity/) and include it in the automated environment install for both Ubuntu and Omarchy.

Description

• Add browser/google-antigravity.yml which installs /usr/local/bin/google-antigravity (uses xdg-open / gio fallback) and a desktop entry at /usr/share/applications/google-antigravity.desktop.
• Invoke the new playbook from install_environment in Makefile so it runs during the main install flow, and update Readme.md to list Google Antigravity among the installed applications.

Testing

• Attempted ansible-playbook browser/google-antigravity.yml -i local --syntax-check but the command failed because ansible-playbook is not installed in this environment, so no automated playbook validation was performed.

---

Codex Task