feat: add Cursor, Gemini CLI, and Antigravity plugin support

[zzAllenn]

Summary

• add the Cursor-native plugin manifest and marketplace metadata with inline AgentKey MCP OAuth
• add the root Gemini CLI extension manifest using Streamable HTTP and native OAuth discovery
• add one root Antigravity plugin package for both Antigravity 2.0 and Antigravity CLI
• configure Antigravity remote MCP with the required serverUrl field and automatic OAuth discovery
• reuse the existing skills/agentkey/ package across Gemini and Antigravity without duplication
• keep endpoint and release/version synchronization across every client
• document Cursor, Gemini, Antigravity 2.0, and Antigravity CLI onboarding in English and Chinese

Validation

• npx -y bats@1.12.0 tests/ (20/20 passing)
• npx -y @google/gemini-cli@0.54.4 extensions validate .
• verified all eight version values match 1.13.1
• verified Claude, Codex, Cursor, Kimi, Gemini, and Antigravity MCP endpoints remain synchronized
• verified the AgentKey endpoint advertises OAuth protected-resource metadata
• verified the branch merges cleanly with chainbase-labs/main

Antigravity validation boundary

The agy binary is not installed in the current environment, so a live CLI install was not run. The plugin layout, manifest fields, serverUrl, OAuth configuration, and shared Skill discovery are covered by automated assertions against the current Antigravity documentation. The documentation-prescribed $schema URL currently returns HTTP 404; the manifest retains the prescribed value while tests enforce the published schema locally.