feat(FN-4070): add workflow revision forking for task retries

Implements workflow revision forking with a new per-project setting, allowing follow-up tasks to inherit a fork of the calling task's workflow rather than the original revision. The feature adds classification logic, fork execution, and the settings UI, with docs and tests covering the full flow.

Fusion-Task-Id: FN-4070

packages/engine/src/__tests__/executor-test-helpers.ts

@@ -271,9 +271,22 @@ export function createMockStore() {
     updateTask: vi.fn().mockResolvedValue({}),
     moveTask: vi.fn().mockResolvedValue({}),
     mergeTask: vi.fn().mockResolvedValue({}),
+    createTask: vi.fn().mockImplementation(async (input: Record<string, unknown>) => ({
+      id: "FN-002",
+      title: input.title,
+      description: input.description,
+      column: "triage",
+      dependencies: input.dependencies ?? [],
+      steps: [],
+      currentStep: 0,
+      log: [],
+      createdAt: new Date().toISOString(),
+      updatedAt: new Date().toISOString(),
+    })),
     logEntry: vi.fn().mockResolvedValue(undefined),
     addTaskComment: vi.fn().mockResolvedValue(undefined),
     parseStepsFromPrompt: vi.fn().mockResolvedValue([]),
+    parseFileScopeFromPrompt: vi.fn().mockResolvedValue([]),
     updateSettings: vi.fn().mockResolvedValue({}),
     getSettings: vi.fn().mockResolvedValue({
       maxConcurrent: 2,