Create CHANGELOG following Keep a Changelog format

Add CHANGELOG.md documenting all implemented features:
- Client with functional options pattern
- Authentication with 2FA and auto token renewal
- HTTP request helper with retry logic
- Data models (Checklist, Task, Note, User, DueDate)
- Error handling with sentinel errors
- Mage build targets
- Comprehensive unit test suite

Follows Keep a Changelog 1.1.0 and Semantic Versioning.

Closes checkvist-api-93m

CHANGELOG.md

@@ -0,0 +1,53 @@
+# Changelog
+
+All notable changes to this project will be documented in this file.
+
+The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
+and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
+
+## [Unreleased]
+
+### Added
+
+- **Client**: Core API client with functional options pattern
+  - `NewClient(username, remoteKey, ...Option)` constructor
+  - `WithHTTPClient`, `WithTimeout`, `WithRetryConfig`, `WithLogger`, `WithBaseURL` options
+  - Thread-safe token management with automatic renewal
+
+- **Authentication**: Full authentication support
+  - `Authenticate(ctx)` for explicit login
+  - `AuthenticateWith2FA(ctx, token)` for 2FA support
+  - Automatic token refresh before expiry
+  - Token sent via `X-Client-Token` header
+
+- **HTTP Request Helper**: Internal request handling with retry logic
+  - Exponential backoff for HTTP 429 (rate limiting) and 5xx errors
+  - Configurable retry settings via `RetryConfig`
+  - Context cancellation support
+  - Debug logging via `slog`
+
+- **Data Models**: Type-safe structs for Checkvist entities
+  - `Checklist` with ID, Name, Public, Archived, TaskCount, etc.
+  - `Task` with Content, Status, Priority, DueDate, Tags, Children
+  - `Note` for task comments
+  - `User` for user information
+  - `TaskStatus` enum (Open, Closed, Invalidated)
+  - `DueDate` with smart syntax support (`DueToday`, `DueTomorrow`, `DueAt`, `DueInDays`)
+
+- **Error Handling**: Structured error types
+  - `APIError` with StatusCode, Message, RequestID
+  - Sentinel errors: `ErrUnauthorized`, `ErrNotFound`, `ErrRateLimited`, `ErrBadRequest`, `ErrServerError`
+  - Compatible with `errors.Is()` and `errors.As()`
+
+- **Build Automation**: Mage build targets
+  - `mage test` - run tests
+  - `mage coverage` - run tests with coverage
+  - `mage lint` - run staticcheck
+  - `mage fmt` - format code
+  - `mage check` - run all quality checks
+
+- **Tests**: Comprehensive unit test suite
+  - Client configuration tests
+  - Authentication flow tests
+  - Retry logic tests
+  - httptest.Server-based mocking