feat: add cursor plugin

[joneidejohnsen]

Summary

• Add Cursor Marketplace plugin packaging (.cursor-plugin/marketplace.json and .cursor-plugin/plugin.json) so the toolkit can be distributed as a Cursor plugin alongside the existing Claude Code plugin
• Add validation script (scripts/validate-cursor-plugin.mjs) and CI step to keep the plugin structure valid
• Add name frontmatter to command files (required by Cursor plugin format)
• Clean up plugin descriptions and README structure

Details

The plugin uses "source": "./" in the marketplace manifest — Cursor's auto-discovery picks up rules/, skills/, commands/, and .mcp.json directly from the repo root. No restructuring into a plugins/ subdirectory needed since there's only one plugin.

New files:

• .cursor-plugin/marketplace.json — marketplace manifest
• .cursor-plugin/plugin.json — per-plugin manifest with explicit mcpServers reference to .mcp.json
• assets/logo.svg — Sanity logo for marketplace display
• scripts/validate-cursor-plugin.mjs — validates marketplace structure and frontmatter

README changes:

• Merged Claude Code and Cursor plugin options into a single "Install plugin" section
• Renamed "context rules" to "agent rules" throughout
• Reordered features list (MCP first)

Test plan

• npm run validate:all passes (skills + Cursor plugin)
• Submit to Cursor Marketplace for review