Adds the Thomson Reuters CoCounsel Legal plugin (Westlaw Deep Research)
as a vendor-maintained plugin under a new external_plugins/ directory,
following the partner-built layout used elsewhere.
- external_plugins/cocounsel-legal/: plugin.json, .mcp.json (HTTP MCP
server with OAuth), deep-research skill, vendor README
- .claude-plugin/marketplace.json: register cocounsel-legal pointing at
./external_plugins/cocounsel-legal with Thomson Reuters as author
- README.md: document external_plugins/ in the repo layout, add an
'External / partner-built' section, list the CoCounsel Legal connector,
and add a skill reference entry
