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 now leave out of their own listing both the manifest they
write and a temp file an interrupted run left beside it, by file
identity (os.SameFile), so each is recognized however its path is
spelled, while an ordinary file of the same name elsewhere in the tree
is still listed.

Model: opus-5-5

internal/cli/check.go

@@ -75,25 +75,22 @@ func safeRateUint64(rate float64) uint64 {
 	return uint64(rate)
 }
 
-// findManifest looks for a manifest file in the given directory.
-// It checks for index.mf and .index.mf, returning the first one found.
+// findManifest returns the path of the manifest with the default name in
+// dir, or an error if there is none.
 func findManifest(fs afero.Fs, dir string) (string, error) {
-	candidates := []string{"index.mf", ".index.mf"}
-	for _, name := range candidates {
-		path := filepath.Join(dir, name)
+	path := filepath.Join(dir, defaultManifestName)
 
-		exists, err := afero.Exists(fs, path)
-		if err != nil {
-			return "", err
-		}
-
-		if exists {
-			return path, nil
-		}
+	exists, err := afero.Exists(fs, path)
+	if err != nil {
+		return "", err
 	}
 
-	return "", fmt.Errorf(
-		"%w in %s (looked for index.mf and .index.mf)", errNoManifestFound, dir)
+	if !exists {
+		return "", fmt.Errorf("%w in %s (looked for %s)",
+			errNoManifestFound, dir, defaultManifestName)
+	}
+
+	return path, nil
 }
 
 // fetchManifestToTemp downloads a manifest URL to a temporary file and