feat(antigravity-auth): add OAuth flow and token management

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src/auth/antigravity/index.ts

@@ -1,5 +1,7 @@
 // Antigravity auth module barrel export
-// Types and constants will be populated by Task 1 and Task 2
 
 export * from "./types"
 export * from "./constants"
+export * from "./oauth"
+export * from "./token"
+export * from "./project"