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

.beads/issues.jsonl

@@ -11,7 +11,7 @@
 {"id":"checkvist-api-8jh","title":"Implement repeating tasks configuration","description":"Add P2 (nice-to-have) repeat support to TaskBuilder:\n- WithRepeat(pattern string) *TaskBuilder\nSupport Checkvist smart syntax for repeats.\nDocument common patterns in GoDoc.","status":"open","priority":2,"issue_type":"feature","owner":"mail@oliverjakoubek.de","created_at":"2026-01-14T12:30:56.826106108+01:00","created_by":"Oliver Jakoubek","updated_at":"2026-01-14T12:30:56.826106108+01:00","dependencies":[{"issue_id":"checkvist-api-8jh","depends_on_id":"checkvist-api-tjk","type":"blocks","created_at":"2026-01-14T12:33:03.159849575+01:00","created_by":"Oliver Jakoubek"}]}
 {"id":"checkvist-api-8q3","title":"Set up Mage build targets","description":"Create magefiles/magefile.go with:\n- Test() - run go test -v ./...\n- Coverage() - run go test -coverprofile=coverage.out ./...\n- Lint() - run staticcheck ./...\n- Fmt() - run gofmt -w .\n- Check() - run all quality checks (fmt, vet, staticcheck, test)\nEnsure magefiles has its own go.mod importing magefile.org/mage","status":"closed","priority":0,"issue_type":"task","owner":"mail@oliverjakoubek.de","created_at":"2026-01-14T12:31:09.228450637+01:00","created_by":"Oliver Jakoubek","updated_at":"2026-01-14T13:33:08.511791793+01:00","closed_at":"2026-01-14T13:33:08.511791793+01:00","close_reason":"Closed","dependencies":[{"issue_id":"checkvist-api-8q3","depends_on_id":"checkvist-api-5wr","type":"blocks","created_at":"2026-01-14T12:32:48.556022687+01:00","created_by":"Oliver Jakoubek"}]}
 {"id":"checkvist-api-8u6","title":"Implement HTTP request helper with retry logic","description":"Add internal HTTP helper to client.go:\n- doRequest(ctx, method, path, body) helper for all API calls\n- Automatic authentication check before requests\n- JSON marshaling/unmarshaling\n- Exponential backoff retry for:\n  - HTTP 429 (Too Many Requests)\n  - HTTP 5xx (Server Errors)\n  - Network errors (timeout, connection reset)\n- Respect context cancellation\n- Optional debug logging of requests/responses via slog","status":"closed","priority":0,"issue_type":"task","owner":"mail@oliverjakoubek.de","created_at":"2026-01-14T12:31:08.780244392+01:00","created_by":"Oliver Jakoubek","updated_at":"2026-01-14T13:27:52.914675409+01:00","closed_at":"2026-01-14T13:27:52.914675409+01:00","close_reason":"Closed","dependencies":[{"issue_id":"checkvist-api-8u6","depends_on_id":"checkvist-api-ymg","type":"blocks","created_at":"2026-01-14T12:32:47.973194416+01:00","created_by":"Oliver Jakoubek"},{"issue_id":"checkvist-api-8u6","depends_on_id":"checkvist-api-mnh","type":"blocks","created_at":"2026-01-14T12:32:48.268500727+01:00","created_by":"Oliver Jakoubek"}]}
-{"id":"checkvist-api-93m","title":"Create CHANGELOG","description":"Create CHANGELOG.md following Keep a Changelog format:\n- [Unreleased] section for ongoing work\n- Initial release preparation notes\n- Document all features implemented","status":"open","priority":0,"issue_type":"task","owner":"mail@oliverjakoubek.de","created_at":"2026-01-14T12:31:39.009748936+01:00","created_by":"Oliver Jakoubek","updated_at":"2026-01-14T12:31:39.009748936+01:00"}
+{"id":"checkvist-api-93m","title":"Create CHANGELOG","description":"Create CHANGELOG.md following Keep a Changelog format:\n- [Unreleased] section for ongoing work\n- Initial release preparation notes\n- Document all features implemented","status":"closed","priority":0,"issue_type":"task","owner":"mail@oliverjakoubek.de","created_at":"2026-01-14T12:31:39.009748936+01:00","created_by":"Oliver Jakoubek","updated_at":"2026-01-14T13:37:00.644317886+01:00","closed_at":"2026-01-14T13:37:00.644317886+01:00","close_reason":"Closed"}
 {"id":"checkvist-api-bbx","title":"Write unit tests for Notes","description":"Create notes_test.go with tests:\n- TestNotes_List\n- TestNotes_Create\n- TestNotes_Update\n- TestNotes_Delete\nUse table-driven tests. Create testdata/notes/ fixtures.","status":"open","priority":0,"issue_type":"task","owner":"mail@oliverjakoubek.de","created_at":"2026-01-14T12:31:37.829382141+01:00","created_by":"Oliver Jakoubek","updated_at":"2026-01-14T12:31:37.829382141+01:00","dependencies":[{"issue_id":"checkvist-api-bbx","depends_on_id":"checkvist-api-5ab","type":"blocks","created_at":"2026-01-14T12:33:14.119755191+01:00","created_by":"Oliver Jakoubek"}]}
 {"id":"checkvist-api-br3","title":"Core API Operations","description":"Phase 2: Implement CRUD operations for Checklists, Tasks, and Notes. All P0 (must-have) features for the library.","status":"open","priority":0,"issue_type":"epic","owner":"mail@oliverjakoubek.de","created_at":"2026-01-14T12:30:53.20627925+01:00","created_by":"Oliver Jakoubek","updated_at":"2026-01-14T12:30:53.20627925+01:00"}
 {"id":"checkvist-api-c2k","title":"Implement Checklist operations","description":"Create checklists.go with ChecklistService:\n- client.Checklists() returns ChecklistService\n- List(ctx) ([]Checklist, error) - GET /checklists.json\n- Get(ctx, id) (*Checklist, error) - GET /checklists/{id}.json\n- Create(ctx, name) (*Checklist, error) - POST /checklists.json\n- Update(ctx, id, name) (*Checklist, error) - PUT /checklists/{id}.json\n- Delete(ctx, id) error - DELETE /checklists/{id}.json\n- Support archived filter in List\nContext support for all methods.","status":"open","priority":0,"issue_type":"feature","owner":"mail@oliverjakoubek.de","created_at":"2026-01-14T12:30:53.566197933+01:00","created_by":"Oliver Jakoubek","updated_at":"2026-01-14T12:30:53.566197933+01:00","dependencies":[{"issue_id":"checkvist-api-c2k","depends_on_id":"checkvist-api-8u6","type":"blocks","created_at":"2026-01-14T12:32:54.533462004+01:00","created_by":"Oliver Jakoubek"},{"issue_id":"checkvist-api-c2k","depends_on_id":"checkvist-api-lpn","type":"blocks","created_at":"2026-01-14T12:32:54.859645166+01:00","created_by":"Oliver Jakoubek"}]}

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