Default the manifest to index.mf and keep it out of its own listing (closes #100)

mfer gen now writes index.mf instead of .index.mf, the name fetch
requests and the README calls the standard filename. Given a
directory, check, freshen, list and export look only for index.mf;
.index.mf is no longer recognized. Because index.mf is not hidden, gen
and freshen leave out of their own listing the manifest they write and
a temp file an interrupted run left beside it, matched by file
identity (os.SameFile) however the path is spelled. The scanner takes
these as a plain ExcludePaths list; manifestTempPath alone names the
temp file, for both writes, both skips and the tests.

Model: opus-5-5

internal/cli/mfer.go

@@ -25,8 +25,19 @@ const (
 	flagProgress = "progress"
 
 	manifestArgsUsage = "[manifest file]"
+
+	// defaultManifestName is the filename gen writes by default, the one
+	// looked for when a command is given a directory, and the one fetch
+	// appends to a directory URL.
+	defaultManifestName = "index.mf"
 )
 
+// manifestTempPath returns the temp file gen and freshen write a manifest
+// to before renaming it to out.
+func manifestTempPath(out string) string {
+	return out + ".tmp"
+}
+
 // errUnknownCommand indicates an unrecognized command argument.
 var errUnknownCommand = errors.New("unknown command")
 
@@ -156,7 +167,7 @@ func (mfa *CLIApp) generateCommand() *cli.Command {
 			},
 			&cli.StringFlag{
 				Name:    "output",
-				Value:   "./.index.mf",
+				Value:   defaultManifestName,
 				Aliases: []string{"o"},
 				Usage:   "Specify output filename",
 			},