feat(FN-3008): notify when model settings fall back to defaults

Merges FN-3008 to add a "fallback-used" notification system: the engine now emits events when AI model fallbacks are triggered, dispatches notifications via ntfy/webhook providers, surfaces a session banner in the dashboard, and exposes a settings toggle to enable or disable these alerts.

Fusion-Task-Id: FN-3008

packages/engine/src/merger.ts

@@ -155,6 +155,7 @@ import {
 import { describeModel, promptWithFallback } from "./pi.js";
 import { accumulateSessionTokenUsage } from "./session-token-usage.js";
 import { createResolvedAgentSession, extractRuntimeHint } from "./agent-session-helpers.js";
+import { notifyFallbackUsed } from "./notifier.js";
 import { buildSessionSkillContext } from "./session-skill-context.js";
 import type { WorktreePool } from "./worktree-pool.js";
 import { AgentLogger } from "./agent-logger.js";
@@ -2336,6 +2337,8 @@ You are assisting with a paused \`git pull --rebase\`.
       ? settings.defaultModelIdOverride
       : settings.defaultModelId,
     defaultThinkingLevel: settings.defaultThinkingLevel,
+    taskId,
+    onFallbackModelUsed: notifyFallbackUsed,
   });
 
   const prompt = [