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/freshen.go

@@ -157,7 +157,7 @@ func (s *freshenScanner) walk(path string, info fs.FileInfo, walkErr error) erro
 	}
 
 	// Skip the manifest file itself
-	if relPath == s.manifestBase || relPath == "."+s.manifestBase {
+	if relPath == s.manifestBase {
 		return nil
 	}