make prompt colors configurable via GOSH_COLORS

- GOSH_COLORS='user=..:root=..:path=..' with ANSI SGR codes,
  empty value disables the color for that part
- only digits and ';' are accepted, invalid entries are ignored
- read before every prompt, so 'set GOSH_COLORS=...' takes effect immediately
- document in help and README, including example color schemes

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

main_test.go

@@ -1026,25 +1026,52 @@ 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 + " $ "
+	got := buildPrompt("flow", "host", "~/src", false, &defaultColors)
+	want := "\x1b[1;32mflow@host\x1b[0m:\x1b[1;34m~/src\x1b[0m $ "
 	if got != want {
 		t.Errorf("buildPrompt() = %q, want %q", got, want)
 	}
 
-	got = buildPrompt("root", "host", "/", true, true)
-	if !strings.HasPrefix(got, colorRoot+"root@host") {
+	got = buildPrompt("root", "host", "/", true, &defaultColors)
+	if !strings.HasPrefix(got, "\x1b[1;31mroot@host") {
 		t.Errorf("expected red user for root, got %q", got)
 	}
 
-	got = buildPrompt("flow", "host", "~/src", false, false)
+	got = buildPrompt("flow", "host", "~/src", false, nil)
 	if got != "flow@host:~/src $ " {
 		t.Errorf("buildPrompt() without color = %q", got)
 	}
+
+	colors := promptColors{user: "38;5;208", path: ""}
+	got = buildPrompt("flow", "host", "~/src", false, &colors)
+	if got != "\x1b[38;5;208mflow@host\x1b[0m:~/src $ " {
+		t.Errorf("buildPrompt() with custom colors = %q", got)
+	}
+}
+
+func TestParseColors(t *testing.T) {
+	tests := []struct {
+		spec string
+		want promptColors
+	}{
+		{"", defaultColors},
+		{"user=1;33", promptColors{user: "1;33", root: "1;31", path: "1;34"}},
+		{"user=33:root=41;97:path=38;5;208", promptColors{user: "33", root: "41;97", path: "38;5;208"}},
+		{"path=", promptColors{user: "1;32", root: "1;31", path: ""}},
+		{"path=0m\x1b[2J", defaultColors}, // escape sequences are rejected
+		{"path=red", defaultColors},       // only numeric codes
+		{"foo=1:user", defaultColors},     // unknown key, missing value
+		{"user=35::path=36", promptColors{user: "35", root: "1;31", path: "36"}},
+	}
+	for _, tc := range tests {
+		if got := parseColors(tc.spec); got != tc.want {
+			t.Errorf("parseColors(%q) = %+v, want %+v", tc.spec, got, tc.want)
+		}
+	}
 }
 
 func TestBuildPrompt_ColorEscapesControlCharacters(t *testing.T) {
-	got := buildPrompt("flow", "host", "evil\x1b[2J\x07", false, true)
+	got := buildPrompt("flow", "host", "evil\x1b[2J\x07", false, &defaultColors)
 	if strings.Contains(got, "\x1b[2J") || strings.Contains(got, "\x07") {
 		t.Errorf("prompt contains control characters from path: %q", got)
 	}