docs(cursor): use published marketplace installation

[tkkhq]

Why

• Volcano is published at https://cursor.com/marketplace/kong/volcano. Recommend Cursor Marketplace while retaining optional manual installation outside the marketplace.

Summary

• link the published listing from README.md and document Customize → Install → project or user scope in plugins/cursor/README.md
• mark Cursor Marketplace as recommended and retain clone-and-run installation as optional
• keep CLI setup separate through /install-volcano and its bundled setup skill
• reuse the existing copy installer for manual installation and local development; remove remote shell piping and unsupported external symlink instructions
• distinguish GitHub team-marketplace Auto Refresh from reviewed public listing updates

Verification

• make validate
• git diff --check origin/main...HEAD
• checked relative documentation links, installation anchors, and recommended-before-optional instruction order
• fetched the published listing and confirmed HTTP 200
• checked installation and local-loading steps against https://cursor.com/docs/plugins#installing-plugins and https://cursor.com/docs/plugins#test-plugins-locally
• tested manual installation from a fresh public clone with an isolated home directory and no initialized submodules; compared the copied manifest and bundled setup skill with the clone

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