Implement Tag API methods (CRITICAL)

Add direct client methods for tag operations:

Must-have methods:
- GetTaskTags(ctx, taskID) - returns map[tagID]tagName
- SetTaskTags(ctx, projectID, taskID, tags) - replaces all tags

Nice-to-have methods:
- GetAllTags(ctx) - list all tags in system
- GetTagsByProject(ctx, projectID) - list project tags
- CreateTag(ctx, projectID, name, colorID) - create new tag
- UpdateTag(ctx, tagID, name, colorID) - update existing tag
- RemoveTag(ctx, tagID) - delete tag

Note: setTaskTags REPLACES all tags. Individual add/remove
requires read-modify-write pattern (to be implemented in TaskScope).

Closes: kanboard-api-16r

tags.go

@@ -0,0 +1,111 @@
+package kanboard
+
+import (
+	"context"
+	"fmt"
+)
+
+// GetTaskTags returns the tags assigned to a task as a map of tagID to tag name.
+func (c *Client) GetTaskTags(ctx context.Context, taskID int) (map[int]string, error) {
+	params := map[string]int{"task_id": taskID}
+
+	// Kanboard returns map[string]string where keys are string tag IDs
+	var result map[string]string
+	if err := c.call(ctx, "getTaskTags", params, &result); err != nil {
+		return nil, fmt.Errorf("getTaskTags: %w", err)
+	}
+
+	// Convert string keys to int
+	tags := make(map[int]string, len(result))
+	for idStr, name := range result {
+		var id int
+		if _, err := fmt.Sscanf(idStr, "%d", &id); err != nil {
+			continue // Skip invalid IDs
+		}
+		tags[id] = name
+	}
+
+	return tags, nil
+}
+
+// SetTaskTags sets the tags for a task, replacing all existing tags.
+// Tags are specified by name. Non-existent tags will be auto-created.
+func (c *Client) SetTaskTags(ctx context.Context, projectID, taskID int, tags []string) error {
+	params := map[string]interface{}{
+		"project_id": projectID,
+		"task_id":    taskID,
+		"tags":       tags,
+	}
+
+	var result bool
+	if err := c.call(ctx, "setTaskTags", params, &result); err != nil {
+		return fmt.Errorf("setTaskTags: %w", err)
+	}
+
+	return nil
+}
+
+// GetAllTags returns all tags in the system.
+func (c *Client) GetAllTags(ctx context.Context) ([]Tag, error) {
+	var result []Tag
+	if err := c.call(ctx, "getAllTags", nil, &result); err != nil {
+		return nil, fmt.Errorf("getAllTags: %w", err)
+	}
+	return result, nil
+}
+
+// GetTagsByProject returns all tags for a specific project.
+func (c *Client) GetTagsByProject(ctx context.Context, projectID int) ([]Tag, error) {
+	params := map[string]int{"project_id": projectID}
+
+	var result []Tag
+	if err := c.call(ctx, "getTagsByProject", params, &result); err != nil {
+		return nil, fmt.Errorf("getTagsByProject: %w", err)
+	}
+	return result, nil
+}
+
+// CreateTag creates a new tag in a project and returns the tag ID.
+func (c *Client) CreateTag(ctx context.Context, projectID int, name, colorID string) (int, error) {
+	params := map[string]interface{}{
+		"project_id": projectID,
+		"tag":        name,
+	}
+	if colorID != "" {
+		params["color_id"] = colorID
+	}
+
+	var result int
+	if err := c.call(ctx, "createTag", params, &result); err != nil {
+		return 0, fmt.Errorf("createTag: %w", err)
+	}
+	return result, nil
+}
+
+// UpdateTag updates an existing tag's name and/or color.
+func (c *Client) UpdateTag(ctx context.Context, tagID int, name, colorID string) error {
+	params := map[string]interface{}{
+		"tag_id": tagID,
+		"tag":    name,
+	}
+	if colorID != "" {
+		params["color_id"] = colorID
+	}
+
+	var result bool
+	if err := c.call(ctx, "updateTag", params, &result); err != nil {
+		return fmt.Errorf("updateTag: %w", err)
+	}
+	return nil
+}
+
+// RemoveTag deletes a tag from the system.
+func (c *Client) RemoveTag(ctx context.Context, tagID int) error {
+	params := map[string]int{"tag_id": tagID}
+
+	var result bool
+	if err := c.call(ctx, "removeTag", params, &result); err != nil {
+		return fmt.Errorf("removeTag: %w", err)
+	}
+	return nil
+}