Add package-level GoDoc documentation

Creates doc.go with comprehensive package documentation including
quick start, authentication, fluent API usage, task creation,
error handling, thread safety, and tag operation warnings.

doc.go

@@ -0,0 +1,67 @@
+// Package kanboard provides a Go client for the Kanboard JSON-RPC API.
+//
+// This library offers both a fluent API for convenient, chainable operations
+// and direct API methods for lower-level access. The client is safe for
+// concurrent use by multiple goroutines.
+//
+// # Quick Start
+//
+// Create a client with API token authentication:
+//
+//	client := kanboard.NewClient("https://kanboard.example.com").
+//	    WithAPIToken("your-api-token")
+//
+// # Authentication
+//
+// The library supports two authentication methods:
+//
+//   - API Token: Use [Client.WithAPIToken] for token-based auth (recommended)
+//   - Basic Auth: Use [Client.WithBasicAuth] for username/password auth
+//
+// # Fluent API
+//
+// Use [Client.Board] for project-scoped operations:
+//
+//	board := client.Board(projectID)
+//	tasks, _ := board.GetTasks(ctx, kanboard.StatusActive)
+//	task, _ := board.CreateTaskFromParams(ctx,
+//	    kanboard.NewTask("Title").WithDescription("Details"))
+//
+// Use [Client.Task] for task-scoped operations:
+//
+//	task := client.Task(taskID)
+//	task.MoveToNextColumn(ctx)
+//	task.AddTag(ctx, "reviewed")
+//	task.AddComment(ctx, userID, "Comment text")
+//
+// # Task Creation
+//
+// Use [TaskParams] for fluent task creation:
+//
+//	params := kanboard.NewTask("Title").
+//	    WithDescription("Details").
+//	    WithPriority(2).
+//	    WithTags("urgent", "backend")
+//
+// # Error Handling
+//
+// The library provides typed errors and helper functions:
+//
+//	task, err := client.GetTask(ctx, taskID)
+//	if kanboard.IsNotFound(err) {
+//	    // Handle not found
+//	}
+//	if kanboard.IsUnauthorized(err) {
+//	    // Handle auth failure
+//	}
+//
+// # Thread Safety
+//
+// The [Client] is safe for concurrent use. Request IDs are generated atomically.
+//
+// # Tag Operations
+//
+// Warning: Kanboard's setTaskTags API replaces all tags. The [TaskScope.AddTag]
+// and [TaskScope.RemoveTag] methods use read-modify-write internally and are
+// not atomic. Concurrent tag modifications may cause data loss.
+package kanboard