docs: fix Antigravity link and add initialization example

Port of upstream PR github#1605 (applied manually due to README structural divergence).
- Fix Antigravity URL: agy.ai → antigravity.google
- Add agy initialization example in Quick Start section

Upstream commit: f00bce4
Co-authored-by: Qiangjun Ran <ranqiangjun@gmail.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: 2 people

README.md

@@ -131,7 +131,7 @@ Purpose:
 | `q` | Amazon Q Developer CLI | CLI | `.amazonq/` |
 | `amp` | Amp | CLI | `.agents/` |
 | `shai` | SHAI | CLI | `.shai/` |
-| `agy` | Antigravity | IDE | `.agent/` |
+| `agy` | [Antigravity](https://antigravity.google/) | IDE | `.agent/` |
 | `bob` | IBM Bob | IDE | `.bob/` |
 
 For per-agent details and install links, see `docs/agents.md` and the live `AGENT_CONFIG` in `src/specify_cli/__init__.py`.
@@ -241,6 +241,12 @@ Use current directory mode if needed:
 specify init . --ai claude --script sh
 ```
 
+Initialize with Antigravity support:
+
+```bash
+specify init my-project --ai agy
+```
+
 ### 2) Run Phase 0 in your agent chat
 
 ```text