Restore files at 0600 and make the blob hash check unskippable (closes #163)

Regular files are now created with O_EXCL at mode 0600 and given their stored mode only after the content is written and closed, so a file whose stored mode is restrictive is never briefly readable by other local users mid-restore. A file whose write or close fails is removed rather than left partial, and a chmod failure is a user-visible warning instead of a debug line.

hashVerifyReader.Close now errors when closed before EOF, so a short read or early close can never obtain a blob whose hash was not verified; downloadBlobToCache drops the cache entry on any such failure.

verifyFile (--verify) now rejects a restored file with bytes past its last chunk. Tests cover each behaviour under umask 022.

Model: opus-4-8

internal/vaultik/blob_fetch_hash_test.go

@@ -133,3 +133,51 @@ func TestFetchAndDecryptBlobVerifiesHash(t *testing.T) {
 		}
 	})
 }
+
+// TestFetchAndDecryptBlobCloseBeforeEOFFails verifies the hash check
+// cannot be skipped: a caller that reads only part of the blob and then
+// closes gets an error rather than silently unverified bytes.
+func TestFetchAndDecryptBlobCloseBeforeEOFFails(t *testing.T) {
+	t.Parallel()
+
+	identity, err := age.GenerateX25519Identity()
+	if err != nil {
+		t.Fatalf("generating identity: %v", err)
+	}
+
+	plaintext := []byte("hello world test data for blob hash verification")
+	encryptedData, correctHash := buildHashTestBlob(t, identity, plaintext)
+
+	mockStorage := NewMockStorer()
+	blobPath := "blobs/" + correctHash[:2] + "/" +
+		correctHash[2:4] + "/" + correctHash
+
+	mockStorage.mu.Lock()
+	mockStorage.data[blobPath] = encryptedData
+	mockStorage.mu.Unlock()
+
+	tv := vaultik.NewForTesting(mockStorage)
+
+	rc, err := tv.FetchAndDecryptBlob(
+		context.Background(), correctHash, int64(len(encryptedData)), identity)
+	if err != nil {
+		t.Fatalf("unexpected error opening stream: %v", err)
+	}
+
+	// Read one byte, far short of the plaintext length, then close.
+	buf := make([]byte, 1)
+
+	_, err = rc.Read(buf)
+	if err != nil {
+		t.Fatalf("reading first byte: %v", err)
+	}
+
+	err = rc.Close()
+	if err == nil {
+		t.Fatal("expected error closing before EOF, got nil")
+	}
+
+	if !strings.Contains(err.Error(), "hash not verified") {
+		t.Fatalf("expected not-verified error, got: %v", err)
+	}
+}