Add update-check endpoint support (v0.2.1)

Implement /update-check endpoint aligned with remote OpenAPI spec:
- Add checkForUpdates() method to LicenseClientInterface
- Add UpdateInfo DTO for update check responses
- Add ProductNotFoundException for product_not_found error
- Update local openapi.json to v0.4.0

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

CLAUDE.md

@@ -29,10 +29,6 @@ This project is proudly **"vibe-coded"** using Claude.AI - the entire codebase w
 
 No known bugs at the moment
 
-### Version 0.2.1
-
-No pending tasks at the moment.
-
 ### Version 0.3.0
 
 No pending tasks at the moment.
@@ -237,3 +233,27 @@ When editing CLAUDE.md or other markdown files, follow these rules to avoid lint
 - Version tagging with `git tag -a` creates annotated tags with messages
 - CHANGELOG.md follows Keep a Changelog format (MD024 duplicate headings are expected)
 - Roadmap in CLAUDE.md should be updated after each release to reflect next versions
+
+### 2026-01-27 - Version 0.2.1 (Update Check Endpoint)
+
+**Completed:**
+
+- Compared local `openapi.json` against remote server specification
+- Discovered new `/update-check` endpoint in remote spec
+- Added `UpdateInfo` DTO for update check responses
+- Added `ProductNotFoundException` for `product_not_found` error code
+- Implemented `checkForUpdates()` method in `LicenseClientInterface`
+- Implemented `checkForUpdates()` in both `LicenseClient` and `SecureLicenseClient`
+- Updated local `openapi.json` with `/update-check` endpoint (now v0.4.0)
+- Updated `LicenseException::fromApiResponse()` to handle `product_not_found`
+- Added encoded constant `ENCODED_UPDATE_CHECK` to `SecureLicenseClient`
+- Updated documentation: README.md and docs/client-implementation.md
+- All 66 tests pass
+
+**Learnings:**
+
+- Remote OpenAPI spec at server repo may have newer endpoints than local copy
+- Update check endpoint returns WordPress-compatible plugin info (tested, requires, requires_php)
+- `UpdateInfo` DTO supports both `download_url` and `package` fields (WordPress compatibility)
+- Package hash format uses `sha256:hexstring` prefix for integrity verification
+- StringEncoder can generate encoded constants for obfuscated endpoint names