chore: add sonnet-4-6 mapping, config defaults, and CI improvements

- Add claude-sonnet-4-6 to default Antigravity model mapping
- Add antigravity_extra_retries default value in config
- Add cache-dependency-path to CI setup-go for faster builds
- Simplify vitest config to avoid vite plugin compatibility issues

.github/workflows/backend-ci.yml

@@ -17,6 +17,7 @@ jobs:
           go-version-file: backend/go.mod
           check-latest: false
           cache: true
+          cache-dependency-path: backend/go.sum
       - name: Verify Go version
         run: |
           go version | grep -q 'go1.25.7'
@@ -36,6 +37,7 @@ jobs:
           go-version-file: backend/go.mod
           check-latest: false
           cache: true
+          cache-dependency-path: backend/go.sum
       - name: Verify Go version
         run: |
           go version | grep -q 'go1.25.7'
@@ -44,4 +46,4 @@ jobs:
         with:
           version: v2.7
           args: --timeout=5m
-          working-directory: backend
+          working-directory: backend