Fix version command and add default action with banner

- Version command now uses mfer.Version constant instead of empty build flags
- Running just 'mfer' shows banner + help
- Unknown commands still return error with exit code 1

internal/cli/entry_test.go

@@ -42,7 +42,7 @@ func TestVersionCommand(t *testing.T) {
 
 	assert.Equal(t, 0, exitCode)
 	stdout := opts.Stdout.(*bytes.Buffer).String()
-	assert.Contains(t, stdout, "1.0.0")
+	assert.Contains(t, stdout, mfer.Version)
 	assert.Contains(t, stdout, "abc123")
 }