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/mfer.go

@@ -23,10 +23,11 @@ const (
 	cmdFetch    = "fetch"
 	cmdVersion  = "version"
 
-	flagProgress         = "progress"
-	flagTimeout          = "timeout"
-	flagDest             = "dest"
-	flagRequireSignature = "require-signature"
+	flagProgress           = "progress"
+	flagTimeout            = "timeout"
+	flagDest               = "dest"
+	flagRequireSignature   = "require-signature"
+	flagIncludePermissions = "include-permissions"
 
 	manifestArgsUsage = "[manifest file]"
 
@@ -167,6 +168,16 @@ func requireSignatureFlag() *cli.StringFlag {
 	}
 }
 
+// includePermissionsFlag returns the --include-permissions flag taken by the
+// generate and freshen subcommands.
+func includePermissionsFlag() *cli.BoolFlag {
+	return &cli.BoolFlag{
+		Name: flagIncludePermissions,
+		Usage: "Record each file's permission bits in manifest " +
+			"(recorded as 0000 by default)",
+	}
+}
+
 func (mfa *CLIApp) generateCommand() *cli.Command {
 	return &cli.Command{
 		Name:         cmdGenerate,
@@ -224,6 +235,7 @@ func (mfa *CLIApp) generateCommand() *cli.Command {
 				Usage: "Include createdAt timestamp in manifest " +
 					"(omitted by default for determinism)",
 			},
+			includePermissionsFlag(),
 		),
 	}
 }
@@ -307,6 +319,7 @@ func (mfa *CLIApp) freshenCommand() *cli.Command {
 				Usage: "Include createdAt timestamp in manifest " +
 					"(omitted by default for determinism)",
 			},
+			includePermissionsFlag(),
 		),
 	}
 }
@@ -350,7 +363,7 @@ func (mfa *CLIApp) listCommand() *cli.Command {
 			&cli.BoolFlag{
 				Name:    "long",
 				Aliases: []string{"l"},
-				Usage:   "Show size and mtime",
+				Usage:   "Show mode, size and mtime",
 			},
 			&cli.BoolFlag{
 				Name:  "print0",