feat(antigravity): add gemini-3.5-flash-extra-low (Low) model

- Add gemini-3.5-flash-extra-low across CLI menu, provider models, usage, pricing
- Add MITM synonyms (high/medium/extra-low) and split pattern so Low no longer falls through to Medium
- Strip models/ prefix in getMappedModel for AG public name normalization

Co-authored-by: Cursor <cursoragent@cursor.com>

cli/src/cli/menus/providers.js

@@ -55,6 +55,7 @@ const PROVIDER_MODELS = {
   ag: [
     { id: "gemini-3-flash-agent" },
     { id: "gemini-3.5-flash-low" },
+    { id: "gemini-3.5-flash-extra-low" },
     { id: "gemini-pro-agent" },
     { id: "gemini-3.1-pro-low" },
     { id: "claude-sonnet-4-6" },