Record file mode in the manifest, 0000 unless asked (closes #161)

MFFilePath gains mode (field 304): a file's permission bits, 0777 at
most, or 0000, meaning none recorded. gen and freshen record real modes
only with --include-permissions (ScannerOptions.IncludePermissions); the
builder keeps only mode.Perm(), so setuid, setgid and sticky are never
written. list -l and export show the mode in octal. check reports
MODE_MISMATCH for a recorded mode other than 0000 the file lacks. fetch
refuses a manifest with a mode above 0777 before requesting any file,
sets only the permission bits of each recorded mode on the files it
writes, and downloads again a present file whose mode differs. The decoding-cost bound counts a file
entry at 176 bytes, up from 160.

Model: opus-5-5

internal/cli/errmsg_test.go

@@ -162,7 +162,7 @@ func signedManifest(t *testing.T, files map[string][]byte) []byte {
 
 	for path, content := range files {
 		_, err = b.AddFile(mfer.RelFilePath(path), mfer.FileSize(len(content)),
-			mfer.ModTime{}, bytes.NewReader(content), nil)
+			mfer.ModTime{}, 0, bytes.NewReader(content), nil)
 		require.NoError(t, err)
 	}