Implement TaskScope tag methods with read-modify-write (CRITICAL)

- GetTags: retrieve task tags as map[int]string
- SetTags: replace all tags on a task
- ClearTags: remove all tags
- AddTag: read-modify-write, idempotent (no-op if tag exists)
- RemoveTag: read-modify-write, idempotent (no-op if tag doesn't exist)
- HasTag: check if task has a specific tag

WARNING: Tag operations are NOT atomic. Concurrent modifications may
cause data loss due to the read-modify-write pattern required by
Kanboard's setTaskTags API.

Comprehensive test coverage including idempotency tests.

task_scope.go

@@ -51,3 +51,115 @@ func (t *TaskScope) MoveToProject(ctx context.Context, projectID int) error {
 func (t *TaskScope) Update(ctx context.Context, params *TaskUpdateParams) error {
 	return t.client.UpdateTaskFromParams(ctx, t.taskID, params)
 }
+
+// GetTags returns the tags assigned to this task as a map of tagID to tag name.
+func (t *TaskScope) GetTags(ctx context.Context) (map[int]string, error) {
+	return t.client.GetTaskTags(ctx, t.taskID)
+}
+
+// SetTags sets the tags for this task, replacing all existing tags.
+// Tags are specified by name. Non-existent tags will be auto-created.
+//
+// WARNING: This operation is not atomic. Concurrent tag modifications may cause data loss.
+func (t *TaskScope) SetTags(ctx context.Context, tags ...string) error {
+	task, err := t.Get(ctx)
+	if err != nil {
+		return err
+	}
+	return t.client.SetTaskTags(ctx, int(task.ProjectID), t.taskID, tags)
+}
+
+// ClearTags removes all tags from this task.
+//
+// WARNING: This operation is not atomic. Concurrent tag modifications may cause data loss.
+func (t *TaskScope) ClearTags(ctx context.Context) error {
+	return t.SetTags(ctx)
+}
+
+// AddTag adds a tag to this task using a read-modify-write pattern.
+// If the tag already exists on the task, this is a no-op (idempotent).
+//
+// WARNING: This operation is not atomic. Concurrent tag modifications may cause data loss.
+func (t *TaskScope) AddTag(ctx context.Context, tag string) error {
+	// Get task info for project_id
+	task, err := t.Get(ctx)
+	if err != nil {
+		return err
+	}
+
+	// Get current tags
+	currentTags, err := t.client.GetTaskTags(ctx, t.taskID)
+	if err != nil {
+		return err
+	}
+
+	// Check if tag already exists (idempotent)
+	for _, existingTag := range currentTags {
+		if existingTag == tag {
+			return nil
+		}
+	}
+
+	// Build new tag list
+	tagNames := make([]string, 0, len(currentTags)+1)
+	for _, name := range currentTags {
+		tagNames = append(tagNames, name)
+	}
+	tagNames = append(tagNames, tag)
+
+	// Set updated tags
+	return t.client.SetTaskTags(ctx, int(task.ProjectID), t.taskID, tagNames)
+}
+
+// RemoveTag removes a tag from this task using a read-modify-write pattern.
+// If the tag doesn't exist on the task, this is a no-op (idempotent).
+//
+// WARNING: This operation is not atomic. Concurrent tag modifications may cause data loss.
+func (t *TaskScope) RemoveTag(ctx context.Context, tag string) error {
+	// Get task info for project_id
+	task, err := t.Get(ctx)
+	if err != nil {
+		return err
+	}
+
+	// Get current tags
+	currentTags, err := t.client.GetTaskTags(ctx, t.taskID)
+	if err != nil {
+		return err
+	}
+
+	// Filter out the tag to remove
+	tagNames := make([]string, 0, len(currentTags))
+	found := false
+	for _, name := range currentTags {
+		if name == tag {
+			found = true
+			continue
+		}
+		tagNames = append(tagNames, name)
+	}
+
+	// If tag wasn't found, nothing to do (idempotent)
+	if !found {
+		return nil
+	}
+
+	// Set updated tags
+	return t.client.SetTaskTags(ctx, int(task.ProjectID), t.taskID, tagNames)
+}
+
+// HasTag checks if this task has a specific tag.
+func (t *TaskScope) HasTag(ctx context.Context, tag string) (bool, error) {
+	tags, err := t.client.GetTaskTags(ctx, t.taskID)
+	if err != nil {
+		return false, err
+	}
+
+	for _, name := range tags {
+		if name == tag {
+			return true, nil
+		}
+	}
+
+	return false, nil
+}