feat: Add a tool to search Linear Issues in copilot (#11518)

This PR adds a tool to search Linear issues. If the integration is
enabled for the account, the tool will return results as expected. Also
introduces support for an `active?` method, which allows third-party
Copilot tools to be conditionally enabled based on the status of the
integration on the account.

enterprise/app/services/captain/tools/base_service.rb

@@ -31,4 +31,8 @@ class Captain::Tools::BaseService
       }
     }
   end
+
+  def active?
+    true
+  end
 end