fix(install): correct Antigravity CLI config paths (2026 unification)

Antigravity support targeted the pre-unification layout, so it never actually
configured the CLI:
- detection probed ~/.gemini/antigravity/ — the CLI now installs under
  ~/.gemini/antigravity-cli/ (brain/, mcp/, settings.json)
- MCP config was written to ~/.gemini/antigravity/mcp_config.json — Antigravity
  reads the SHARED ~/.gemini/config/mcp_config.json (mcpServers, command/args)
- the SessionStart hook + AGENTS.md targeted the stale dir

Point detection at ~/.gemini/antigravity-cli/, write the MCP server to the
shared ~/.gemini/config/mcp_config.json (creating ~/.gemini/config if needed),
and place AGENTS.md + the SessionStart reminder under ~/.gemini/antigravity-cli/.
Install, uninstall, install --plan, the detection test, and the README table
updated.