FN-6193: guard merger settings lookup for deleted tasks

Keep AI merge commit finalization running when the task disappears during settings resolution.

- fall back to a synthetic task stub before merging effective settings in runAiAgentForCommit
- add an engine regression test covering deleted-task settings degradation
- add a patch changeset for the published CLI package

Files changed:
 .changeset/fn-6193-merger-deleted-task.md          |  5 +++
 .../src/__tests__/merger-settings-degrade.test.ts  | 43 ++++++++++++++++++++++
 packages/engine/src/merger.ts                      |  3 +-
 3 files changed, 50 insertions(+), 1 deletion(-)

Fusion-Task-Id: FN-6193

Fusion-Task-Lineage: 1d63d24e-1321-450c-95c9-ffc058fd8387

packages/engine/src/__tests__/merger-settings-degrade.test.ts

@@ -0,0 +1,43 @@
+import { describe, expect, it, vi } from "vitest";
+
+import { BUILTIN_WORKFLOW_SETTINGS, type Settings } from "@fusion/core";
+import { mergeEffectiveSettings } from "../effective-settings.js";
+
+const PROJECT = "proj-1";
+
+function baseSettings(): Settings {
+  const catalogDefaults = Object.fromEntries(
+    BUILTIN_WORKFLOW_SETTINGS
+      .filter((setting) => setting.default !== undefined)
+      .map((setting) => [setting.id, setting.default]),
+  );
+  return {
+    ...catalogDefaults,
+    persistAgentToolOutput: false,
+    executionProvider: "project-anthropic",
+    executionModelId: "claude-project",
+  } as unknown as Settings;
+}
+
+describe("merger settings degrade path", () => {
+  it("uses a fallback task id so deleted-task getTask failures still return base settings", async () => {
+    const taskId = "FN-6193-DELETED";
+    const store = {
+      getTask: vi.fn(async () => {
+        throw new Error(`Task ${taskId} not found`);
+      }),
+      getTaskWorkflowSelection: vi.fn(() => {
+        throw new Error(`Task ${taskId} not found`);
+      }),
+      getWorkflowDefinition: vi.fn(async () => undefined),
+      getWorkflowSettingValues: vi.fn(() => ({})),
+      getWorkflowSettingsProjectId: vi.fn(() => PROJECT),
+    };
+    const base = baseSettings();
+
+    const taskForSettings = await store.getTask(taskId).catch(() => ({ id: taskId } as const));
+    const merged = await mergeEffectiveSettings(store as any, taskForSettings, base);
+
+    expect(merged).toEqual(base);
+  });
+});

packages/engine/src/merger.ts

@@ -11679,9 +11679,10 @@ async function runAiAgentForCommit(params: AiAgentParams): Promise<{ success: bo
   // Merge per-task effective workflow settings (U3, KTD-3) — this worker re-fetches
   // settings independently of aiMergeTask, so apply the same merge here (covers the
   // titleSummarizer lane reads in resolveSafeCommitBody). Behavior-inert by default.
+  const taskForSettings = await store.getTask(taskId).catch(() => ({ id: taskId } as const));
   const settings = await mergeEffectiveSettings(
     store,
-    await store.getTask(taskId),
+    taskForSettings,
     await store.getSettings(),
   );