feat(antigravity): add thought signature support for multi-turn conversations

Gemini 3 Pro requires thoughtSignature on function call blocks in
subsequent requests. This commit:

- Add thought-signature-store for session-based signature storage
- Extract signature from both streaming (SSE) and non-streaming responses
- Inject signature into functionCall parts on subsequent requests
- Maintain consistent sessionId per fetch instance

Debug logging available via ANTIGRAVITY_DEBUG=1

🤖 GENERATED WITH ASSISTANCE OF [OhMyOpenCode](https://github.com/code-yeongyu/oh-my-opencode)

src/auth/antigravity/index.ts

@@ -1,5 +1,3 @@
-// Antigravity auth module barrel export
-
 export * from "./types"
 export * from "./constants"
 export * from "./oauth"
@@ -9,5 +7,6 @@ export * from "./request"
 export * from "./response"
 export * from "./tools"
 export * from "./thinking"
+export * from "./thought-signature-store"
 export * from "./fetch"
 export * from "./plugin"