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/errmsg_test.go

@@ -74,7 +74,7 @@ func TestNoManifestFoundMessage(t *testing.T) {
 	_, err := findManifest(afero.NewMemMapFs(), "/tmp/x")
 	require.ErrorIs(t, err, errNoManifestFound)
 	assert.EqualError(t, err,
-		"no manifest found in /tmp/x (looked for index.mf and .index.mf)")
+		"no manifest found in /tmp/x (looked for index.mf)")
 }
 
 func TestVerifyRequiredSignerMessages(t *testing.T) {