feat(cli): add NO_COLOR environment variable support

Implement the NO_COLOR standard (https://no-color.org/) for CLI output.
When NO_COLOR is set (to any value), ANSI color codes are disabled.

Changes:
- Add init() to check NO_COLOR and TERM=dumb environment variables
- Add ColorEnabled() to query current color state
- Add SetColorEnabled() to programmatically enable/disable colors
- Modify AnsiStyle.Render() to return plain text when colors disabled
- Update UseASCII() to also disable colors (consistent with ASCII mode)
- Add comprehensive tests for color enable/disable functionality

Usage:
  NO_COLOR=1 core dev status  # Runs without color output
  TERM=dumb core dev status   # Also disables colors

Closes #87

Co-Authored-By: Claude <noreply@anthropic.com>

pkg/cli/ansi.go

@@ -2,8 +2,10 @@ package cli
 
 import (
 	"fmt"
+	"os"
 	"strconv"
 	"strings"
+	"sync"
 )
 
 // ANSI escape codes
@@ -15,6 +17,40 @@ const (
 	ansiUnderline = "\033[4m"
 )
 
+var (
+	colorEnabled   = true
+	colorEnabledMu sync.RWMutex
+)
+
+func init() {
+	// NO_COLOR standard: https://no-color.org/
+	// If NO_COLOR is set (to any value, including empty), disable colors.
+	if _, exists := os.LookupEnv("NO_COLOR"); exists {
+		colorEnabled = false
+		return
+	}
+
+	// TERM=dumb indicates a terminal without color support.
+	if os.Getenv("TERM") == "dumb" {
+		colorEnabled = false
+	}
+}
+
+// ColorEnabled returns true if ANSI color output is enabled.
+func ColorEnabled() bool {
+	colorEnabledMu.RLock()
+	defer colorEnabledMu.RUnlock()
+	return colorEnabled
+}
+
+// SetColorEnabled enables or disables ANSI color output.
+// This overrides the NO_COLOR environment variable check.
+func SetColorEnabled(enabled bool) {
+	colorEnabledMu.Lock()
+	colorEnabled = enabled
+	colorEnabledMu.Unlock()
+}
+
 // AnsiStyle represents terminal text styling.
 // Use NewStyle() to create, chain methods, call Render().
 type AnsiStyle struct {
@@ -68,8 +104,9 @@ func (s *AnsiStyle) Background(hex string) *AnsiStyle {
 }
 
 // Render applies the style to text.
+// Returns plain text if NO_COLOR is set or colors are disabled.
 func (s *AnsiStyle) Render(text string) string {
-	if s == nil {
+	if s == nil || !ColorEnabled() {
 		return text
 	}
 

pkg/cli/ansi_test.go

@@ -6,6 +6,10 @@ import (
 )
 
 func TestAnsiStyle_Render(t *testing.T) {
+	// Ensure colors are enabled for this test
+	SetColorEnabled(true)
+	defer SetColorEnabled(true) // Reset after test
+
 	s := NewStyle().Bold().Foreground("#ff0000")
 	got := s.Render("test")
 	if got == "test" {
@@ -18,3 +22,76 @@ func TestAnsiStyle_Render(t *testing.T) {
 		t.Error("Output should contain bold code")
 	}
 }
+
+func TestColorEnabled_Good(t *testing.T) {
+	// Save original state
+	original := ColorEnabled()
+	defer SetColorEnabled(original)
+
+	// Test enabling
+	SetColorEnabled(true)
+	if !ColorEnabled() {
+		t.Error("ColorEnabled should return true")
+	}
+
+	// Test disabling
+	SetColorEnabled(false)
+	if ColorEnabled() {
+		t.Error("ColorEnabled should return false")
+	}
+}
+
+func TestRender_ColorDisabled_Good(t *testing.T) {
+	// Save original state
+	original := ColorEnabled()
+	defer SetColorEnabled(original)
+
+	// Disable colors
+	SetColorEnabled(false)
+
+	s := NewStyle().Bold().Foreground("#ff0000")
+	got := s.Render("test")
+
+	// Should return plain text without ANSI codes
+	if got != "test" {
+		t.Errorf("Expected plain 'test', got %q", got)
+	}
+}
+
+func TestRender_ColorEnabled_Good(t *testing.T) {
+	// Save original state
+	original := ColorEnabled()
+	defer SetColorEnabled(original)
+
+	// Enable colors
+	SetColorEnabled(true)
+
+	s := NewStyle().Bold()
+	got := s.Render("test")
+
+	// Should contain ANSI codes
+	if !strings.Contains(got, "\033[") {
+		t.Error("Expected ANSI codes when colors enabled")
+	}
+}
+
+func TestUseASCII_Good(t *testing.T) {
+	// Save original state
+	original := ColorEnabled()
+	defer SetColorEnabled(original)
+
+	// Enable first, then UseASCII should disable colors
+	SetColorEnabled(true)
+	UseASCII()
+	if ColorEnabled() {
+		t.Error("UseASCII should disable colors")
+	}
+}
+
+func TestRender_NilStyle_Good(t *testing.T) {
+	var s *AnsiStyle
+	got := s.Render("test")
+	if got != "test" {
+		t.Errorf("Nil style should return plain text, got %q", got)
+	}
+}

pkg/cli/glyph.go

@@ -25,8 +25,11 @@ func UseUnicode() { currentTheme = ThemeUnicode }
 // UseEmoji switches the glyph theme to Emoji.
 func UseEmoji() { currentTheme = ThemeEmoji }
 
-// UseASCII switches the glyph theme to ASCII.
-func UseASCII() { currentTheme = ThemeASCII }
+// UseASCII switches the glyph theme to ASCII and disables colors.
+func UseASCII() {
+	currentTheme = ThemeASCII
+	SetColorEnabled(false)
+}
 
 func glyphMap() map[string]string {
 	switch currentTheme {