v1.108.89 — correct Antigravity config docs (revert 1.108.88 installer)

Corrects 1.108.88, whose install antigravity target wrote to the wrong config locations and therefore did not actually configure Antigravity.

Changed

• Reverted the install antigravity target (and its client auto-detection, alias, and skill wiring) from 1.108.88. cli/init.py / cli/skills.py / server.py are restored byte-for-byte to their 1.108.87 shape.
• Documented the correct manual config. Antigravity (2.0 / IDE / CLI) loads MCP servers from the shared HOME-level ~/.gemini/config/mcp_config.json (mcpServers key); shared agent skills live in ~/.gemini/skills/. A copy-paste block is in the README "Works with" section.

Antigravity has worked via manual config since #307 was closed (2026-05-23); this just documents it at the right path. A first-class installer remains deferred pending demand. No INDEX_VERSION bump.