test(smoke): update Antigravity E2E to 2026 CLI layout

The Phase 8 agent-install E2E checked the pre-unification Antigravity paths
(~/.gemini/antigravity/), which broke after the config-path fix. Point the
stub setup at ~/.gemini/antigravity-cli/ and assert the MCP server lands in
the shared ~/.gemini/config/mcp_config.json with AGENTS.md under
~/.gemini/antigravity-cli/. Verified end-to-end (smoke ALL PASSED).