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, the
scanner now leaves the output file out of the listing by its path: gen
--force over an existing manifest no longer lists the old one, while a
file named index.mf in a subdirectory is still listed.

Model: opus-5-5

internal/cli/manifest_loader.go

@@ -65,7 +65,7 @@ func (mfa *CLIApp) openManifestReader(pathOrURL string) (io.ReadCloser, error) {
 }
 
 // resolveManifestArg resolves the manifest path from CLI arguments.
-// HTTP(S) URLs are returned as-is. Directories are searched for index.mf/.index.mf.
+// HTTP(S) URLs are returned as-is. Directories are searched for index.mf.
 // If no argument is given, the current directory is searched.
 func (mfa *CLIApp) resolveManifestArg(ctx *cli.Context) (string, error) {
 	if ctx.Args().Len() > 0 {