feat(06-01): add Formatter interface, Registry, and TTY color detection

- pkg/output/formatter.go: Formatter interface, Options, Registry with
  Register/Get/Names, ErrUnknownFormat sentinel
- pkg/output/colors.go: IsTTY + ColorsEnabled honoring NO_COLOR
- Promote github.com/mattn/go-isatty to direct dependency
- Unit tests cover registry round-trip, unknown lookup, sorted Names,
  non-TTY buffer, NO_COLOR override

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

pkg/output/colors_test.go

@@ -0,0 +1,37 @@
+package output
+
+import (
+	"bytes"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestColorsEnabled_BufferIsNotTTY(t *testing.T) {
+	t.Setenv("NO_COLOR", "")
+	// Explicitly unset NO_COLOR so the non-TTY branch is exercised.
+	t.Setenv("NO_COLOR", "")
+	assert.False(t, ColorsEnabled(&bytes.Buffer{}), "bytes.Buffer must never be considered a TTY")
+}
+
+func TestColorsEnabled_NoColorEnvForcesOff(t *testing.T) {
+	t.Setenv("NO_COLOR", "1")
+	assert.False(t, ColorsEnabled(&bytes.Buffer{}))
+}
+
+func TestColorsEnabled_NilWriterSafe(t *testing.T) {
+	t.Setenv("NO_COLOR", "")
+	// A nil *os.File should return false, not panic.
+	var f *nilWriter
+	assert.False(t, ColorsEnabled(f))
+}
+
+// nilWriter is a typed-nil io.Writer used to verify ColorsEnabled does not
+// panic when passed a non-*os.File writer.
+type nilWriter struct{}
+
+func (*nilWriter) Write(p []byte) (int, error) { return len(p), nil }
+
+func TestIsTTY_NilFile(t *testing.T) {
+	assert.False(t, IsTTY(nil))
+}