Honor NO_COLOR in the terminal log handler (closes #402)

The TTY log handler always wrote ANSI color, unlike internal/ui, which
disables color when NO_COLOR is set. Add a color flag to TTYHandler,
set from NO_COLOR with the same test internal/ui uses, and route every
color code through a paint helper that returns plain text when color is
off. The README's color sentence now covers log lines too.

Model: deepseek-v4-pro

internal/log/tty_handler_test.go

@@ -421,3 +421,19 @@ func TestTTYHandlerEmptyGroupAndAttrsAreNoOps(t *testing.T) {
 	assert.Contains(t, plain, "kept=yes")
 	assert.NotContains(t, plain, " =")
 }
+
+// TestTTYHandlerRespectsNoColor checks that the handler writes no ANSI
+// color when NO_COLOR is set, matching internal/ui's shouldColor test.
+// The handler is only ever chosen for a terminal, so this is the one
+// remaining condition that can still leave color in log output.
+func TestTTYHandlerRespectsNoColor(t *testing.T) {
+	t.Setenv("NO_COLOR", "1")
+
+	var buf bytes.Buffer
+
+	logger := slog.New(log.NewTTYHandler(&buf, debugHandlerOptions()))
+	logger.Info("hello", "key", "value")
+
+	assert.NotContains(t, buf.String(), "\x1b[",
+		"NO_COLOR must suppress every ANSI escape in log output")
+}