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.go

@@ -0,0 +1,34 @@
+package output
+
+import (
+	"io"
+	"os"
+
+	"github.com/mattn/go-isatty"
+)
+
+// IsTTY reports whether f is an open terminal or Cygwin/MSYS pty.
+// Returns false for a nil file.
+func IsTTY(f *os.File) bool {
+	if f == nil {
+		return false
+	}
+	fd := f.Fd()
+	return isatty.IsTerminal(fd) || isatty.IsCygwinTerminal(fd)
+}
+
+// ColorsEnabled reports whether ANSI color output should be emitted on w.
+// Returns false when:
+//   - the NO_COLOR environment variable is set (https://no-color.org/), or
+//   - w is not an *os.File (e.g. bytes.Buffer, strings.Builder), or
+//   - w is an *os.File but not a terminal.
+func ColorsEnabled(w io.Writer) bool {
+	if _, ok := os.LookupEnv("NO_COLOR"); ok {
+		return false
+	}
+	f, ok := w.(*os.File)
+	if !ok {
+		return false
+	}
+	return IsTTY(f)
+}