Fix docs: update Antigravity link and add initialization example (github#1748)

* Fix docs: update Antigravity link and add initialization example

* Update README.md

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

---------

Co-authored-by: Manfred Riem <15701806+mnriem@users.noreply.github.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: 3 people

README.md

@@ -161,7 +161,7 @@ Want to see Spec Kit in action? Watch our [video overview](https://www.youtube.c
 | [Roo Code](https://roocode.com/)                                                     | ✅      |                                                                                                                                           |
 | [SHAI (OVHcloud)](https://github.com/ovh/shai)                                       | ✅      |                                                                                                                                           |
 | [Windsurf](https://windsurf.com/)                                                    | ✅      |                                                                                                                                           |
-| [Antigravity (agy)](https://agy.ai/)                                                 | ✅      |                                                                                                                                           |
+| [Antigravity (agy)](https://antigravity.google/)                                     | ✅      |                                                                                                                                           |
 | Generic                                                                              | ✅      | Bring your own agent — use `--ai generic --ai-commands-dir <path>` for unsupported agents                                                 |
 
 ## 🔧 Specify CLI Reference
@@ -222,6 +222,9 @@ specify init my-project --ai shai
 # Initialize with IBM Bob support
 specify init my-project --ai bob
 
+# Initialize with Antigravity support
+specify init my-project --ai agy
+
 # Initialize with an unsupported agent (generic / bring your own agent)
 specify init my-project --ai generic --ai-commands-dir .myagent/commands/