fix(memory): handle ENOENT gracefully in readFile instead of throwing

When a memory file doesn't exist yet (e.g. daily log `2026-02-19.md`),
`readFile` now returns `{ text: "", path }` instead of propagating the
ENOENT error. This prevents noisy error responses from the memory read
tool and aligns with the "graceful degradation" recommendation in #9307.

Closes #9307

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/agents/tools/memory-tool.e2e.test.ts

@@ -12,7 +12,7 @@ let searchImpl: () => Promise<unknown[]> = async () => [
     source: "memory" as const,
   },
 ];
-let readFileImpl: () => Promise<string> = async () => "";
+let readFileImpl: () => Promise<unknown> = async () => "";
 
 const stubManager = {
   search: vi.fn(async () => await searchImpl()),
@@ -59,7 +59,7 @@ beforeEach(() => {
       source: "memory" as const,
     },
   ];
-  readFileImpl = async () => "";
+  readFileImpl = async () => ""; // default: return empty string
   vi.clearAllMocks();
 });
 
@@ -189,4 +189,23 @@ describe("memory tools", () => {
       error: "path required",
     });
   });
+
+  it("returns empty text without error when file does not exist (ENOENT)", async () => {
+    readFileImpl = async () => {
+      return { text: "", path: "memory/2026-02-19.md" };
+    };
+
+    const cfg = { agents: { list: [{ id: "main", default: true }] } };
+    const tool = createMemoryGetTool({ config: cfg });
+    expect(tool).not.toBeNull();
+    if (!tool) {
+      throw new Error("tool missing");
+    }
+
+    const result = await tool.execute("call_enoent", { path: "memory/2026-02-19.md" });
+    expect(result.details).toEqual({
+      text: "",
+      path: "memory/2026-02-19.md",
+    });
+  });
 });