feat(FN-2493): add plugin add alias and normalize install guidance

- Route "fn plugin add" to the plugin install handler and update CLI help/usage text to document the alias
- Add bin routing tests that cover plugin add/install parity and updated error/help messaging
- Update getting-started, settings reference, and runtime plugin READMEs to standardize plugin installation examples
- Add a patch changeset for @runfusion/fusion and apply DirectoryPicker/PluginManager design-token styling refinements

packages/dashboard/app/components/PluginManager.css

@@ -43,7 +43,7 @@
   border-radius: var(--radius-sm);
   background: var(
     --bg-tertiary,
-    color-mix(in srgb, var(--text-muted) 18%, transparent)
+    color-mix(in srgb, var(--text-muted) 12%, transparent)
   );
   font-size: 0.85em;
 }