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/memory/manager.ts

@@ -437,7 +437,19 @@ export class MemoryIndexManager extends MemoryManagerEmbeddingOps implements Mem
     if (!absPath.endsWith(".md")) {
       throw new Error("path required");
     }
-    const stat = await fs.lstat(absPath);
+    let stat;
+    try {
+      stat = await fs.lstat(absPath);
+    } catch (err: unknown) {
+      if (
+        err instanceof Error &&
+        "code" in err &&
+        (err as NodeJS.ErrnoException).code === "ENOENT"
+      ) {
+        return { text: "", path: relPath };
+      }
+      throw err;
+    }
     if (stat.isSymbolicLink() || !stat.isFile()) {
       throw new Error("path required");
     }