Add interactive Markdown checklist toggles in the ticket preview.

Co-authored-by: Cursor <cursoragent@cursor.com>

internal/ticket/checklist_test.go

@@ -0,0 +1,65 @@
+package ticket
+
+import (
+	"testing"
+)
+
+func TestListCheckboxes(t *testing.T) {
+	src := `# Steps
+
+- [ ] one
+- [x] two
+* [X] three
+  - [ ] nested
+
+Not a box: [ ] plain
+`
+	got := ListCheckboxes(src)
+	if len(got) != 4 {
+		t.Fatalf("len=%d want 4: %+v", len(got), got)
+	}
+	if got[0].Text != "one" || got[0].Checked || got[0].Line != 2 {
+		t.Fatalf("first: %+v", got[0])
+	}
+	if !got[1].Checked || got[1].Text != "two" {
+		t.Fatalf("second: %+v", got[1])
+	}
+	if !got[2].Checked || got[2].Text != "three" {
+		t.Fatalf("third: %+v", got[2])
+	}
+	if got[3].Text != "nested" || got[3].Checked || got[3].Line != 5 {
+		t.Fatalf("nested: %+v", got[3])
+	}
+}
+
+func TestToggleCheckbox(t *testing.T) {
+	src := "- [ ] a\n- [x] b\n"
+	out, err := ToggleCheckbox(src, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if out != "- [x] a\n- [x] b\n" {
+		t.Fatalf("toggle 0: %q", out)
+	}
+	out, err = ToggleCheckbox(out, 1)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if out != "- [x] a\n- [ ] b\n" {
+		t.Fatalf("toggle 1: %q", out)
+	}
+	if _, err := ToggleCheckbox(src, 9); err == nil {
+		t.Fatal("expected error for bad index")
+	}
+}
+
+func TestToggleCheckboxPreservesIndentAndBullet(t *testing.T) {
+	src := "  * [X] deep"
+	out, err := ToggleCheckbox(src, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if out != "  * [ ] deep" {
+		t.Fatalf("got %q", out)
+	}
+}