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 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/export_test.go

@@ -132,7 +132,7 @@ func TestListFromHTTPURL(t *testing.T) {
 
 	exitCode := runCLI(&RunOptions{
 		Appname: testApp,
-		Args:    []string{testApp, "list", server.URL + "/index.mf"},
+		Args:    []string{testApp, cmdList, server.URL + "/index.mf"},
 		Stdin:   &bytes.Buffer{},
 		Stdout:  &stdout,
 		Stderr:  &stderr,