Add Agent Plugins starter template

[joiscoding]

Summary

• Add starter-agent-plugins, a minimal Agent Plugins sample with a root plugin.json, a skill, and a remote MCP server.
• Document both Cursor Plugin and Agent Plugin formats in the README and docs/add-a-plugin.md.
• Teach scripts/validate-template.mjs to detect and validate Agent Plugins so the new starter is not required to have .cursor-plugin/plugin.json.

Test plan

• Run node scripts/validate-template.mjs and confirm it passes
• Confirm plugins/starter-agent-plugins/ has root plugin.json and mcp.json (no .cursor-plugin/)
• Confirm .cursor-plugin/marketplace.json lists starter-agent-plugins

Made with Cursor