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 +421,44 @@ func TestTTYHandlerEmptyGroupAndAttrsAreNoOps(t *testing.T) {
 	assert.Contains(t, plain, "kept=yes")
 	assert.NotContains(t, plain, " =")
 }
+
+// TestTTYHandlerRespectsNoColor checks that only a non-empty NO_COLOR
+// turns color off, 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) {
+	cases := []struct {
+		name    string
+		noColor string
+		colored bool
+	}{
+		{
+			name:    "NO_COLOR turns color off",
+			noColor: "1",
+		},
+		{
+			name:    "empty NO_COLOR keeps color on",
+			noColor: "",
+			colored: true,
+		},
+	}
+
+	for _, testCase := range cases {
+		t.Run(testCase.name, func(t *testing.T) {
+			t.Setenv("NO_COLOR", testCase.noColor)
+
+			var buf bytes.Buffer
+
+			logger := slog.New(log.NewTTYHandler(&buf, debugHandlerOptions()))
+			logger.Info("hello", "key", "value")
+
+			if testCase.colored {
+				assert.Contains(t, buf.String(), "\x1b[",
+					"an empty NO_COLOR must leave ANSI color in log output")
+			} else {
+				assert.NotContains(t, buf.String(), "\x1b[",
+					"NO_COLOR must suppress every ANSI escape in log output")
+			}
+		})
+	}
+}