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.go

@@ -18,6 +18,7 @@ type ExportEntry struct {
 	Hashes []string `json:"hashes"`
 	Mtime  *string  `json:"mtime,omitempty"`
 	Ctime  *string  `json:"ctime,omitempty"`
+	Mode   string   `json:"mode"` // octal, "0000" when none was recorded
 }
 
 func (mfa *CLIApp) exportManifestOperation(
@@ -49,6 +50,7 @@ func (mfa *CLIApp) exportManifestOperation(
 			Path:   f.GetPath(),
 			Size:   f.GetSize(),
 			Hashes: make([]string, 0, len(f.GetHashes())),
+			Mode:   fmt.Sprintf("%04o", f.GetMode()),
 		}
 
 		for _, h := range f.GetHashes() {