fix: use the documented Antigravity MCP config path

[luojiyin1987]

Problem

The Antigravity integration writes MCP settings to ~/.antigravity/mcp.json. It also identifies Antigravity as an Anthropic IDE.

Google documents Antigravity as its IDE. The documented global MCP file is ~/.gemini/config/mcp_config.json. Server entries use the mcpServers key.

Documentation: https://antigravity.google/docs/mcp

Acceptance criteria

• The installer writes ~/.gemini/config/mcp_config.json.
• The uninstaller removes the wigolo entry from that file.
• Detection and status paths use the documented directory.
• The mcpServers.wigolo key remains unchanged.
• Tests cover the documented path.
• The stale TODO is removed.