feat(phase6): docs, lint fixes, STATE.json update

- Fix lint errors in plugin-client.js (unused var, empty block)
- Update README with plugin architecture and env vars
- Update STATE.json to v4.1 IMPLEMENTATION_COMPLETE
- All 96 tests passing, 0 lint errors

src/plugin-client.js

@@ -43,7 +43,6 @@ function PluginClient(opts) {
  * @returns {Promise<boolean>}
  */
 PluginClient.prototype.isHealthy = function () {
-  var self = this;
   return this._request('GET', '/api/v1/health', null)
     .then(function (data) {
       return data && data.status === 'healthy';
@@ -132,7 +131,7 @@ PluginClient.prototype._request = function (method, apiPath, body) {
           }
         } else {
           var msg = 'HTTP ' + res.statusCode;
-          try { msg = JSON.parse(data).error || msg; } catch (_e) {}
+          try { msg = JSON.parse(data).error || msg; } catch (_e) { /* ignore parse errors */ }
           var err = new Error(msg);
           err.statusCode = res.statusCode;
           reject(err);