add colored prompt

- user@host in bold green (bold red for root), directory in bold blue
- colors are disabled with NO_COLOR, TERM=dumb or when stdout is not a terminal
- parts are sanitized before adding the color codes
- only replace the home directory with ~ on a path boundary

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

main_test.go

@@ -1004,6 +1004,7 @@ func TestSanitize(t *testing.T) {
 }
 
 func TestGetPrompt_EscapesControlCharacters(t *testing.T) {
+	t.Setenv("NO_COLOR", "1")
 	origDir, _ := os.Getwd()
 	defer os.Chdir(origDir)
 
@@ -1024,6 +1025,41 @@ func TestGetPrompt_EscapesControlCharacters(t *testing.T) {
 	}
 }
 
+func TestBuildPrompt_Color(t *testing.T) {
+	got := buildPrompt("flow", "host", "~/src", false, true)
+	want := colorUser + "flow@host" + colorReset + ":" + colorPath + "~/src" + colorReset + " $ "
+	if got != want {
+		t.Errorf("buildPrompt() = %q, want %q", got, want)
+	}
+
+	got = buildPrompt("root", "host", "/", true, true)
+	if !strings.HasPrefix(got, colorRoot+"root@host") {
+		t.Errorf("expected red user for root, got %q", got)
+	}
+
+	got = buildPrompt("flow", "host", "~/src", false, false)
+	if got != "flow@host:~/src $ " {
+		t.Errorf("buildPrompt() without color = %q", got)
+	}
+}
+
+func TestBuildPrompt_ColorEscapesControlCharacters(t *testing.T) {
+	got := buildPrompt("flow", "host", "evil\x1b[2J\x07", false, true)
+	if strings.Contains(got, "\x1b[2J") || strings.Contains(got, "\x07") {
+		t.Errorf("prompt contains control characters from path: %q", got)
+	}
+	if !strings.Contains(got, `evil\x1b[2J\x07`) {
+		t.Errorf("expected escaped path in prompt, got %q", got)
+	}
+}
+
+func TestUseColor_NoColor(t *testing.T) {
+	t.Setenv("NO_COLOR", "1")
+	if useColor() {
+		t.Error("expected no color when NO_COLOR is set")
+	}
+}
+
 func TestFilePathCompleter_SkipsControlCharacters(t *testing.T) {
 	dir := t.TempDir()
 	for _, name := range []string{"good", "go\x1b[31mred"} {