feat: add Octen Grok Build plugin

[dongxu0413]

Summary

• Adds the official Octen Grok Build plugin from the Octen-Team/octen-grok-plugin repository.
• Provides live web search, news search, broad search, URL extraction, and Beta image/video search through the octen-mcp package.
• Pins the source to commit SHA f707f44c0f011392ea6e563b8c9a150289525367 and regenerates the plugin index.

Validation

• Plugin static validation passed.
• Grok Build plugin manifest validation passed.
• Grok Build MCP doctor, in a clean temporary environment, completed a successful handshake and discovered 6 tools.
• Marketplace catalog validation passed.
• Marketplace generated-index check passed.
• Git whitespace check passed.

Security

• Source is published from the official Octen-Team organization.
• The plugin uses one stdio MCP server, no hooks, and reads OCTEN_API_KEY only from the user's local environment.
• Image Search (Octen Design) and Video Search are Beta capabilities.

[dongxu0413]

Superseded by #150, which is submitted from the Octen-Team organization fork and includes the complete reviewer checklist.