feat(FN-5359): add push-to-origin button and hook to merge advance notice

Adds a push-to-origin workflow to the merge notice system, introducing a new `useMergeAdvanceNotice` hook, a `merge-advance-push-origin` route handler, and corresponding UI affordance in the `MergeAdvanceNotice` banner component. The engine gains TOCTOU and refusal audit assertions, and coverage exp

Fusion-Task-Id: FN-5359

Co-authored-by: Fusion (runfusion.ai) <noreply@runfusion.ai>
Fusion-Task-Id: FN-5359

packages/dashboard/src/server.ts

@@ -258,6 +258,7 @@ export interface ServerOptions {
   selfHealingManager?: {
     rootDir: string;
     reconcileInReviewBranchRebind: (opts?: { includeTaskIds?: Set<string> }) => Promise<import("@fusion/engine").RebindResult>;
+    getActiveMergeTaskId: () => string | null;
   };
   /** Optional PluginStore for plugin management routes */
   pluginStore?: import("@fusion/core").PluginStore;
@@ -592,6 +593,7 @@ export function createServer(store: TaskStore, options?: ServerOptions): ReturnT
           selfHealingManager: {
             rootDir: engine.getWorkingDirectory(),
             reconcileInReviewBranchRebind: selfHealing.reconcileInReviewBranchRebind.bind(selfHealing),
+            getActiveMergeTaskId: selfHealing.getActiveMergeTaskId.bind(selfHealing),
           },
         };
       }