feat(gemini): add Gemini 3.6 Flash tier routing and 3.5 Flash Lite

Add gemini-3.6-flash tiered (high/medium/low) for Antigravity routing
via upstreamModelId "gemini-3.6-flash-tiered(level)" + thinkingLevel,
plus gemini-3.6-flash and gemini-3.5-flash-lite direct API models.

- getModelUpstreamId: split (level) suffix before lookup, re-append after
- Antigravity executor: preserve transformed body.model
- MITM extractModel: parse thinkingLevel for tiered model (default medium)
- Isolate Cloud Code endpoints: discovery (loadCodeAssist/onboardUser/
  quota) on PROD cloudcode-pa, chat transport on daily-cloudcode-pa
  to bypass prod 429

tests/unit/antigravity-retry-hook.test.js

@@ -67,8 +67,8 @@ describe("antigravity computeRetryDelay hook (D3)", () => {
     expect(out.request.tools[0].functionDeclarations.map(fn => fn.name)).toEqual(["read_file"]);
   });
 
-  it("registry uses the official IDE cloudcode host and user agent", () => {
-    expect(antigravity.transport.baseUrls).toEqual(["https://cloudcode-pa.googleapis.com"]);
+  it("registry uses the daily IDE cloudcode host and user agent", () => {
+    expect(antigravity.transport.baseUrls).toEqual(["https://daily-cloudcode-pa.googleapis.com"]);
     expect(antigravity.transport.headers["User-Agent"]).toBe("antigravity/ide/2.1.1 darwin/arm64");
   });