feat(antigravity): add Gemini 3.7 Flash models (#3286, #3281)

Add gemini-3.7-flash and its tiered high/medium/low variants to the
Antigravity and Gemini registries, with matching capabilities, pricing
and Antigravity quota tracking.

extractModel now recognises gemini-3.7-flash-tiered alongside 3.6 and
derives the version from the request, so thinkingLevel still maps to the
right tiered alias.

Closes #3286
Closes #3281

cli/src/cli/menus/providers.js

@@ -53,6 +53,9 @@ const PROVIDER_MODELS = {
     { id: "glm-4.7" },
   ],
   ag: [
+    { id: "gemini-3.7-flash-high" },
+    { id: "gemini-3.7-flash-medium" },
+    { id: "gemini-3.7-flash-low" },
     { id: "gemini-3.6-flash-high" },
     { id: "gemini-3.6-flash-medium" },
     { id: "gemini-3.6-flash-low" },