fix: add antigravity to platform-configs targets

Fixes #813. The platform-configs module was missing antigravity from its
targets array, causing all 13+ dependent modules to be skipped when
installing with --target antigravity --profile full.

manifests/install-modules.json

@@ -90,6 +90,7 @@
       "targets": [
         "claude",
         "cursor",
+        "antigravity",
         "codex",
         "opencode"
       ],