Warn that Auto falls back to prompting when unavailable

Claude Code starts in its prompting mode when auto isn't available for the
session's model or backend, so a headless Auto task can still stall. Say so
in the task editor, the task runner comment, and HOW-TO-USE.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

app/src/components/projects/home/TaskEditorModal.test.tsx

@@ -165,10 +165,11 @@ describe("TaskEditorModal", () => {
     expect(screen.queryByText(/cannot answer a permission prompt/i)).toBeNull();
   });
 
-  it("tells Auto mode that blocked actions are denied, not prompted", async () => {
+  it("tells Auto mode that blocked actions are denied, and warns of the fallback", async () => {
     await renderEditor(null, { ...baseProject, permission_mode: "auto" });
     expect(screen.queryByText(/cannot answer a permission prompt/i)).toBeNull();
     expect(screen.getByText(/blocks are denied/i)).toBeInTheDocument();
+    expect(screen.getByText(/falls back to prompting/i)).toBeInTheDocument();
   });
 
   it("spells out the stall risk in any non-Bypass mode", async () => {