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

cli/src/cli/menus/providers.js

@@ -53,6 +53,9 @@ const PROVIDER_MODELS = {
     { id: "glm-4.7" },
   ],
   ag: [
+    { id: "gemini-3.6-flash-high" },
+    { id: "gemini-3.6-flash-medium" },
+    { id: "gemini-3.6-flash-low" },
     { id: "gemini-3-flash-agent" },
     { id: "gemini-3.5-flash-low" },
     { id: "gemini-3.5-flash-extra-low" },
@@ -95,6 +98,8 @@ const PROVIDER_MODELS = {
     { id: "claude-3-5-sonnet-20241022" },
   ],
   gemini: [
+    { id: "gemini-3.6-flash" },
+    { id: "gemini-3.5-flash-lite" },
     { id: "gemini-3-pro-preview" },
     { id: "gemini-2.5-pro" },
     { id: "gemini-2.5-flash" },