Add update-check endpoint documentation (v0.2.2)

- Add /update-check endpoint documentation to server-implementation.md
- Add product_not_found error code to error codes table
- Add handleUpdateCheck() handler example in WordPress plugin
- Add findProduct() method stub for product lookups
- Verified client implementation aligns with server documentation

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

CHANGELOG.md

@@ -7,6 +7,20 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+## [0.2.2] - 2026-01-28
+
+### Added
+
+- `/update-check` endpoint documentation in server-implementation.md
+- `product_not_found` error code to error codes table
+- `handleUpdateCheck()` handler example in WordPress plugin implementation
+- `findProduct()` method stub for product lookups
+
+### Changed
+
+- Verified client implementation aligns with updated server documentation
+- All signature algorithms, key derivation, and JSON canonicalization match server
+
 ## [0.2.1] - 2026-01-27
 
 ### Added