Implement Note operations

Add notes.go with NoteService for task comment CRUD:
- client.Notes(checklistID, taskID) returns NoteService
- List(ctx) - get all notes on a task (GET /comments.json)
- Create(ctx, comment) - add new note (POST /comments.json)
- Update(ctx, noteID, comment) - update note text (PUT /comments/{id}.json)
- Delete(ctx, noteID) - remove note (DELETE /comments/{id}.json)

All methods support context for cancellation and timeouts.

Closes checkvist-api-5ab

notes.go

@@ -1,3 +1,75 @@
 package checkvist
 
+import (
+	"context"
+	"fmt"
+)
+
 // notes.go contains the NoteService for CRUD operations on notes (comments) attached to tasks.
+
+// NoteService provides operations on notes (comments) attached to a specific task.
+type NoteService struct {
+	client      *Client
+	checklistID int
+	taskID      int
+}
+
+// Notes returns a NoteService for performing note operations on the specified task.
+func (c *Client) Notes(checklistID, taskID int) *NoteService {
+	return &NoteService{
+		client:      c,
+		checklistID: checklistID,
+		taskID:      taskID,
+	}
+}
+
+// List returns all notes (comments) attached to the task.
+func (s *NoteService) List(ctx context.Context) ([]Note, error) {
+	path := fmt.Sprintf("/checklists/%d/tasks/%d/comments.json", s.checklistID, s.taskID)
+
+	var notes []Note
+	if err := s.client.doGet(ctx, path, &notes); err != nil {
+		return nil, err
+	}
+	return notes, nil
+}
+
+// createNoteRequest is the request body for creating a note.
+type createNoteRequest struct {
+	Comment string `json:"comment"`
+}
+
+// Create creates a new note (comment) on the task.
+func (s *NoteService) Create(ctx context.Context, comment string) (*Note, error) {
+	path := fmt.Sprintf("/checklists/%d/tasks/%d/comments.json", s.checklistID, s.taskID)
+	body := createNoteRequest{Comment: comment}
+
+	var note Note
+	if err := s.client.doPost(ctx, path, body, &note); err != nil {
+		return nil, err
+	}
+	return &note, nil
+}
+
+// updateNoteRequest is the request body for updating a note.
+type updateNoteRequest struct {
+	Comment string `json:"comment"`
+}
+
+// Update updates an existing note's comment text.
+func (s *NoteService) Update(ctx context.Context, noteID int, comment string) (*Note, error) {
+	path := fmt.Sprintf("/checklists/%d/tasks/%d/comments/%d.json", s.checklistID, s.taskID, noteID)
+	body := updateNoteRequest{Comment: comment}
+
+	var note Note
+	if err := s.client.doPut(ctx, path, body, &note); err != nil {
+		return nil, err
+	}
+	return &note, nil
+}
+
+// Delete permanently deletes a note.
+func (s *NoteService) Delete(ctx context.Context, noteID int) error {
+	path := fmt.Sprintf("/checklists/%d/tasks/%d/comments/%d.json", s.checklistID, s.taskID, noteID)
+	return s.client.doDelete(ctx, path)
+}