feat(plugins): add PluginContext, PluginRegistry, and SettingsStore

- PluginContext: scoped API with crash-safe command wrappers
- PluginRegistry: lifecycle management with graceful init failure
- SettingsStore: plugin-scoped key/value via IPC backend
- Export hooks: pre/post hooks on registry for cross-plugin integration

Amit Haridas

src/plugins/settings-store.js

@@ -0,0 +1,23 @@
+class SettingsStore {
+  /**
+   * @param {object} backend - { get(key), set(key, value) } backed by main process store
+   */
+  constructor(backend) {
+    this.backend = backend;
+  }
+
+  get(key) {
+    return this.backend.get(key);
+  }
+
+  set(key, value) {
+    this.backend.set(key, value);
+  }
+
+  onChanged(key, callback) {
+    // Deferred: plugins read settings on init/activate for MVP.
+    // Full change notification requires IPC watcher in main process.
+  }
+}
+
+module.exports = { SettingsStore };