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/formatter_test.go

@@ -0,0 +1,72 @@
+package output
+
+import (
+	"bytes"
+	"errors"
+	"io"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"github.com/salvacybersec/keyhunter/pkg/engine"
+)
+
+// fakeFormatter is a minimal Formatter used to verify the registry round-trip.
+type fakeFormatter struct{ tag string }
+
+func (f fakeFormatter) Format(findings []engine.Finding, w io.Writer, opts Options) error {
+	_, err := w.Write([]byte(f.tag))
+	return err
+}
+
+func TestFormatter_RegisterAndGet(t *testing.T) {
+	Register("__test_fake__", fakeFormatter{tag: "ok"})
+	t.Cleanup(func() { delete(registry, "__test_fake__") })
+
+	f, err := Get("__test_fake__")
+	require.NoError(t, err)
+	require.NotNil(t, f)
+
+	var buf bytes.Buffer
+	require.NoError(t, f.Format(nil, &buf, Options{}))
+	assert.Equal(t, "ok", buf.String())
+}
+
+func TestFormatter_GetUnknown(t *testing.T) {
+	_, err := Get("__definitely_not_registered__")
+	require.Error(t, err)
+	assert.True(t, errors.Is(err, ErrUnknownFormat), "error should wrap ErrUnknownFormat")
+}
+
+func TestFormatter_NamesSorted(t *testing.T) {
+	Register("__z_fake__", fakeFormatter{})
+	Register("__a_fake__", fakeFormatter{})
+	t.Cleanup(func() {
+		delete(registry, "__z_fake__")
+		delete(registry, "__a_fake__")
+	})
+
+	names := Names()
+	// The full registry may contain other entries (e.g. "table"); just assert
+	// that our two names appear in sorted order relative to each other.
+	var aIdx, zIdx = -1, -1
+	for i, n := range names {
+		switch n {
+		case "__a_fake__":
+			aIdx = i
+		case "__z_fake__":
+			zIdx = i
+		}
+	}
+	require.GreaterOrEqual(t, aIdx, 0)
+	require.GreaterOrEqual(t, zIdx, 0)
+	assert.Less(t, aIdx, zIdx, "Names() must return entries in ascending order")
+}
+
+func TestOptions_Defaults(t *testing.T) {
+	var opts Options
+	assert.False(t, opts.Unmask)
+	assert.Equal(t, "", opts.ToolName)
+	assert.Equal(t, "", opts.ToolVersion)
+}