Add Claude Code marketplace catalog

[hopeatina]

Summary

• add a self-hosted Claude Code marketplace catalog for the OrgX plugin
• document the marketplace add/install commands in the README
• extend plugin verification to validate the marketplace catalog against the manifest

Verification

• npm ci --include=dev
• npm run typecheck
• npm test
• npm run verify
• claude plugin validate .
• git diff --check

Notes

• This does not submit the plugin to Anthropic's central directory; that remains blocked on explicit submitter authorization and directory terms acceptance.