Add interactive Markdown checklist toggles in the ticket preview.

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

internal/ticket/checklist.go

@@ -0,0 +1,63 @@
+package ticket
+
+import (
+	"fmt"
+	"regexp"
+	"strings"
+)
+
+// taskCheckboxRe matches GFM/YouTrack-style task list items:
+//
+//	- [ ] unchecked
+//	- [x] checked
+//	* [X] also accepted
+var taskCheckboxRe = regexp.MustCompile(`^(\s*)([-*+])\s+\[([ xX])\](\s*)(.*)$`)
+
+// Checkbox is one task-list item inside a Markdown description.
+type Checkbox struct {
+	Line    int    // 0-based line index in the source
+	Checked bool
+	Text    string // label after the checkbox marker
+}
+
+// ListCheckboxes returns all task-list items in markdown source order.
+func ListCheckboxes(markdown string) []Checkbox {
+	if markdown == "" {
+		return nil
+	}
+	lines := strings.Split(markdown, "\n")
+	var out []Checkbox
+	for i, line := range lines {
+		m := taskCheckboxRe.FindStringSubmatch(line)
+		if m == nil {
+			continue
+		}
+		out = append(out, Checkbox{
+			Line:    i,
+			Checked: m[3] == "x" || m[3] == "X",
+			Text:    m[5],
+		})
+	}
+	return out
+}
+
+// ToggleCheckbox flips the checked state of the checkbox at index
+// (0-based among ListCheckboxes results) and returns the updated markdown.
+func ToggleCheckbox(markdown string, index int) (string, error) {
+	boxes := ListCheckboxes(markdown)
+	if index < 0 || index >= len(boxes) {
+		return markdown, fmt.Errorf("checkbox %d: not found", index)
+	}
+	lines := strings.Split(markdown, "\n")
+	lineIdx := boxes[index].Line
+	m := taskCheckboxRe.FindStringSubmatch(lines[lineIdx])
+	if m == nil {
+		return markdown, fmt.Errorf("checkbox %d: line changed", index)
+	}
+	mark := " "
+	if !(m[3] == "x" || m[3] == "X") {
+		mark = "x"
+	}
+	lines[lineIdx] = m[1] + m[2] + " [" + mark + "]" + m[4] + m[5]
+	return strings.Join(lines, "\n"), nil
+}