Reject --due "" with --starts-on; add starts_on smoke assertion

Move the clearDue+startsOn conflict check after clear detection so
--due "" --starts-on "value" is caught alongside --no-due --starts-on.
Add missing starts_on assertion to the field preservation smoke test.

Author: jeremy

e2e/smoke/smoke_todos_write.bats

@@ -116,6 +116,7 @@ setup_file() {
   assert_success
   assert_json_value '.data.content' 'Renamed title'
   assert_json_value '.data.due_on' '2026-06-15'
+  assert_json_value '.data.starts_on' '2026-06-01'
   assert_json_value '.data.description' 'Original description'
 
   # Clean up

internal/commands/todos.go

@@ -996,16 +996,17 @@ Clear a field by passing its --no- flag or an empty value:
 			if noDescription && strings.TrimSpace(description) != "" {
 				return output.ErrUsage("--no-description and --description cannot be used together")
 			}
-			if noDue && strings.TrimSpace(startsOn) != "" {
-				return output.ErrUsage("--no-due and --starts-on cannot be used together (Basecamp requires a due date when a start date is set)")
-			}
-
 			// Detect clear intent: explicit --no-X flag or empty value via --X ""
 			clearDue := noDue || (cmd.Flags().Changed("due") && strings.TrimSpace(due) == "")
 			clearStarts := noStartsOn || (cmd.Flags().Changed("starts-on") && strings.TrimSpace(startsOn) == "")
 			clearDescription := noDescription || (cmd.Flags().Changed("description") && strings.TrimSpace(description) == "")
 			needsClear := clearDue || clearStarts || clearDescription
 
+			// Clearing due while setting starts is contradictory (Basecamp enforces starts <= due)
+			if clearDue && strings.TrimSpace(startsOn) != "" {
+				return output.ErrUsage("cannot clear due date and set start date together (Basecamp requires a due date when a start date is set)")
+			}
+
 			// Positional title: args[1:] joined
 			positionalTitle := strings.Join(args[1:], " ")
 

internal/commands/todos_test.go

@@ -1635,7 +1635,16 @@ func TestTodosUpdateConflictingNoDueAndStartsOn(t *testing.T) {
 	cmd := NewTodosCmd()
 	err := executeTodosCommand(cmd, app, "update", "999", "--no-due", "--starts-on", "next monday")
 	require.Error(t, err)
-	assert.Contains(t, err.Error(), "--no-due and --starts-on cannot be used together")
+	assert.Contains(t, err.Error(), "cannot clear due date and set start date together")
+}
+
+func TestTodosUpdateConflictingEmptyDueAndStartsOn(t *testing.T) {
+	app, _ := setupTodosTestApp(t)
+
+	cmd := NewTodosCmd()
+	err := executeTodosCommand(cmd, app, "update", "999", "--due", "", "--starts-on", "next monday")
+	require.Error(t, err)
+	assert.Contains(t, err.Error(), "cannot clear due date and set start date together")
 }
 
 func TestTodosUpdateClearWithSetCombined(t *testing.T) {