feat(FN-5624): suppress transient task.json ENOENT with guard, logging, and

Implements graceful suppression of transient `task.json` ENOENT errors in the executor, logging a suppression signal and surfacing a banner in the UI, with test coverage for both the executor behavior and notification service. Documentation in `docs/architecture.md` and a changeset for `@runfusion/f

Fusion-Task-Id: FN-5624

Co-authored-by: Fusion (runfusion.ai) <noreply@runfusion.ai>
Fusion-Task-Id: FN-5624

packages/engine/src/__tests__/executor-missing-task-json-transient.test.ts

@@ -0,0 +1,29 @@
+import { describe, expect, it } from "vitest";
+import { TaskDeletedError } from "@fusion/core";
+import { isTransientMissingTaskJsonError } from "../executor.js";
+
+describe("isTransientMissingTaskJsonError", () => {
+  const task = {
+    id: "FN-5624",
+    worktree: "/tmp/worktrees/fn-5624",
+  };
+
+  it("matches ENOENT on worktree-scoped .fusion/tasks/<id>/task.json", () => {
+    const err = `ENOENT: no such file or directory, open '/tmp/worktrees/fn-5624/.fusion/tasks/FN-5624/task.json'`;
+    expect(isTransientMissingTaskJsonError(err, task)).toBe(true);
+  });
+
+  it("does not match task.json parse failures", () => {
+    const err = "Failed to parse task.json at /tmp/worktrees/fn-5624/.fusion/tasks/FN-5624/task.json: Unexpected token";
+    expect(isTransientMissingTaskJsonError(err, task)).toBe(false);
+  });
+
+  it("does not match TaskDeletedError", () => {
+    expect(isTransientMissingTaskJsonError(new TaskDeletedError("FN-5624", new Date().toISOString()), task)).toBe(false);
+  });
+
+  it("does not match ENOENT for non-task.json paths", () => {
+    const err = `ENOENT: no such file or directory, open '/tmp/worktrees/fn-5624/.fusion/tasks/FN-5624/PROMPT.md'`;
+    expect(isTransientMissingTaskJsonError(err, task)).toBe(false);
+  });
+});