feat: add Grok Build harness and plugin for OpenDia

[jett22JOE]

Summary

Adds first-class Grok Build support so OpenDia is installable as a harness plugin (not only via manual MCP JSON), matching the Compatible AI Clients invitation in the README.

What changed

• Plugin package at plugins/opendia/:
  • .mcp.json + plugin.json → npx -y opendia
  • Skills: opendia (runtime browser playbook), opendia-harness (setup / doctor / troubleshooting)
  • Plugin README with install paths, tool map, and OpenDia vs cloud-browser guidance
• Marketplace indexes: .grok-plugin/marketplace.json and Claude-compatible .claude-plugin/marketplace.json
• Docs: Grok Build under Compatible AI Clients + full setup section in README; Unreleased CHANGELOG; CONTRIBUTING note for harness plugins
• npm keywords: grok, grok-build on opendia-mcp

Install (after merge)

grok plugin marketplace add aeonfun/opendia
grok plugin install opendia --trust

Or manual:

[mcp_servers.opendia]
command = npx
args = [-y, opendia]
enabled = true
startup_timeout_sec = 60

Scope / non-goals

• No MCP server or browser-extension behavior changes — packaging + docs only
• Extension still required and must show Connected to the local WebSocket

Verification

• grok plugin validate plugins/opendia → valid
• Docs-only / packaging PR; existing MCP + extension CI paths unchanged

Test plan

• Review plugin layout under plugins/opendia/
• Confirm marketplace JSON points at ./plugins/opendia
• Optional: grok plugin install ./plugins/opendia --trust from a checkout and grok mcp doctor opendia with the extension connected