feat(FN-1133): add plugin hot-reload support

- Add PluginLoader hot-load/unload with watch mode, auto-recovery, and staged loading
- Add PluginRunner reactive integration with executor dynamic tools registration
- Add dashboard reload endpoint (POST /api/plugins/reload) and PluginManager UI
- Add comprehensive tests for plugin-hot-reload (core) and plugin-runner (engine)
- Update plugin authoring docs and add memory notes
- Add changeset for @gsxdsm/fusion minor release

packages/dashboard/app/api.ts

@@ -3837,3 +3837,10 @@ export async function updatePluginSettings(
     body: JSON.stringify({ settings }),
   });
 }
+
+/** Reload a running plugin with updated code */
+export async function reloadPlugin(id: string, projectId?: string): Promise<PluginInstallation> {
+  return api<PluginInstallation>(withProjectId(`/plugins/${encodeURIComponent(id)}/reload`, projectId), {
+    method: "POST",
+  });
+}

packages/dashboard/app/components/PluginManager.tsx

@@ -10,8 +10,8 @@
  */
 
 import { useState, useEffect, useCallback } from "react";
-import { Package, Settings, Trash2, Plus, X, RefreshCw } from "lucide-react";
-import { fetchPlugins, installPlugin, enablePlugin, disablePlugin, uninstallPlugin, fetchPluginSettings, updatePluginSettings } from "../api";
+import { Package, Settings, Trash2, Plus, X, RefreshCw, RotateCcw } from "lucide-react";
+import { fetchPlugins, installPlugin, enablePlugin, disablePlugin, uninstallPlugin, fetchPluginSettings, updatePluginSettings, reloadPlugin } from "../api";
 import type { PluginInstallation } from "@fusion/core";
 import type { ToastType } from "../hooks/useToast";
 
@@ -34,6 +34,7 @@ export function PluginManager({ addToast, projectId }: PluginManagerProps) {
   const [showInstall, setShowInstall] = useState(false);
   const [installPath, setInstallPath] = useState("");
   const [installing, setInstalling] = useState(false);
+  const [reloadingPluginId, setReloadingPluginId] = useState<string | null>(null);
   const [selectedPlugin, setSelectedPlugin] = useState<PluginInstallation | null>(null);
   const [pluginSettings, setPluginSettings] = useState<Record<string, unknown>>({});
   const [settingsLoading, setSettingsLoading] = useState(false);
@@ -94,6 +95,19 @@ export function PluginManager({ addToast, projectId }: PluginManagerProps) {
     }
   };
 
+  const handleReload = async (plugin: PluginInstallation) => {
+    try {
+      setReloadingPluginId(plugin.id);
+      await reloadPlugin(plugin.id, projectId);
+      addToast(`${plugin.name} reloaded`, "success");
+      await loadPlugins();
+    } catch (err) {
+      addToast(`Failed to reload plugin: ${err instanceof Error ? err.message : String(err)}`, "error");
+    } finally {
+      setReloadingPluginId(null);
+    }
+  };
+
   const handleUninstall = async (plugin: PluginInstallation) => {
     if (!confirm(`Are you sure you want to uninstall "${plugin.name}"?`)) {
       return;
@@ -230,6 +244,16 @@ export function PluginManager({ addToast, projectId }: PluginManagerProps) {
           </div>
 
           <div className="plugin-detail-actions">
+            {selectedPlugin.state === "started" && (
+              <button
+                className="btn-secondary"
+                onClick={() => handleReload(selectedPlugin)}
+                disabled={reloadingPluginId === selectedPlugin.id}
+              >
+                <RotateCcw size={14} className={reloadingPluginId === selectedPlugin.id ? "spin" : ""} />
+                {reloadingPluginId === selectedPlugin.id ? "Reloading..." : "Reload"}
+              </button>
+            )}
             {selectedPlugin.enabled ? (
               <button className="btn-secondary" onClick={() => handleDisable(selectedPlugin)}>
                 Disable
@@ -303,6 +327,16 @@ export function PluginManager({ addToast, projectId }: PluginManagerProps) {
                 </span>
               </div>
               <div className="plugin-actions">
+                {plugin.state === "started" && (
+                  <button
+                    className="btn-icon"
+                    onClick={() => handleReload(plugin)}
+                    disabled={reloadingPluginId === plugin.id}
+                    title="Reload"
+                  >
+                    <RotateCcw size={14} className={reloadingPluginId === plugin.id ? "spin" : ""} />
+                  </button>
+                )}
                 <label className="toggle-switch">
                   <input
                     type="checkbox"

packages/dashboard/src/plugin-routes.ts

@@ -35,6 +35,7 @@ import {
 
 // PluginRunner interface for optional plugin runner
 interface PluginRunner {
+  reloadPlugin?(pluginId: string): Promise<void>;
   getPluginRoutes(): Array<{ pluginId: string; route: import("@fusion/core").PluginRouteDefinition }>;
 }
 
@@ -229,6 +230,46 @@ export function createPluginRouter(
     res.json(plugin);
   }));
 
+  /**
+   * POST /plugins/:id/reload
+   * Reload a running plugin with updated code.
+   */
+  router.post("/:id/reload", catchHandler(async (req: Request, res: Response) => {
+    const id = req.params.id as string;
+
+    // Validate plugin exists
+    let plugin;
+    try {
+      plugin = await pluginStore.getPlugin(id);
+    } catch (err: unknown) {
+      if ((err as NodeJS.ErrnoException).code === "ENOENT") {
+        throw notFound(`Plugin "${id}" not found`);
+      }
+      throw internalError(err instanceof Error ? err.message : "Unknown error");
+    }
+
+    // Validate plugin is started (must be loaded to reload)
+    if (plugin.state !== "started") {
+      throw badRequest("Plugin is not currently loaded. Use enable instead.");
+    }
+
+    // Check if pluginRunner is available and has reloadPlugin method
+    if (!pluginRunner || !pluginRunner.reloadPlugin) {
+      throw internalError("Plugin runner not available");
+    }
+
+    // Reload the plugin
+    try {
+      await pluginRunner.reloadPlugin(id);
+    } catch (reloadErr) {
+      throw internalError(`Reload failed: ${reloadErr instanceof Error ? reloadErr.message : String(reloadErr)}`);
+    }
+
+    // Return updated plugin
+    const updatedPlugin = await pluginStore.getPlugin(id);
+    res.json(updatedPlugin);
+  }));
+
   /**
    * DELETE /plugins/:id
    * Uninstall a plugin.