fix(xerrors): rename MarshalJSON field platformCode → platform_code

JSON tag on the anonymous struct inside MarshalJSON corrected from
`json:"platformCode,omitempty"` to `json:"platform_code,omitempty"` to
match the OpenAPI spec snake_case convention. All error responses that
serialize *Err directly now emit "platform_code" instead of "platformCode".
PR.md added to .gitignore.

CHANGELOG.md

@@ -5,6 +5,15 @@ All notable changes to this module will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this module adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [1.0.2] - 2026-05-22
+
+### Fixed
+
+- `MarshalJSON` JSON tag corrected from `"platformCode"` to `"platform_code"` to
+  match the OpenAPI spec snake_case convention.
+
+[1.0.2]: https://code.nochebuena.dev/go/xerrors/compare/v1.0.1...v1.0.2
+
 ## [1.0.0] — 2026-05-08
 
 ### Added