Add CoCounsel Legal partner plugin under external_plugins/

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

.claude-plugin/marketplace.json

@@ -99,6 +99,14 @@
       "author": {
         "name": "Anthropic"
       }
+    },
+    {
+      "name": "cocounsel-legal",
+      "source": "./external_plugins/cocounsel-legal",
+      "description": "CoCounsel Legal delivers comprehensive Westlaw Deep Research reports with inline, linked citations to Westlaw and Practical Law sources.",
+      "author": {
+        "name": "Thomson Reuters"
+      }
     }
   ]
 }