Add Google Antigravity plugin compatibility

[anaspk]

Overview

Adds support for Google Antigravity (successor to Gemini CLI) while maintaining compatibility with existing Agent Plugins clients (Claude Code, Codex, etc.).

Changes

Added

• mcp_config.json at repo root with Antigravity-specific MCP server configuration using serverUrl field (required by Antigravity, vs. url in standard mcp.json)
• README.md: New "Antigravity" installation section with global and workspace-level install instructions for both Antigravity CLI and IDE
• README.md: Added mcp_config.json to the "Structure" file tree

Unchanged

• plugin.json — empirically verified to work as-is with Antigravity's lenient parser
• mcp.json — retained for other Agent Plugin-compatible clients
• skills/ directory — already compatible with Antigravity's expected format
• No new nested folders created; follows "repo root is the plugin" pattern

Installation Pattern

Uses direct clone-into-place install (entire repo becomes the plugin), mirroring the existing Codex approach where marketplace.json points to repo root ("path": "./").

Verification Needed

Before merging, please verify:

• Plugin is detected by Antigravity
• All 4 skills appear as slash commands
• Upwork MCP server connects successfully via mcp_config.json
• Antigravity doesn't conflict with other root-level files (.claude-plugin/, .agents/, etc.)
• Nested marketplace.json situation doesn't cause issues

References

• Antigravity Plugin Docs
• Agent Plugins Specification