docs: prepare repository for open-source collaboration

CHANGELOG.md

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+# 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 follows [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
+
+## [Unreleased]
+
+### Added
+
+- Open-source baseline documentation:
+  - `README.md` expanded for setup, architecture, release, and security notes
+  - `LICENSE` (MIT)
+  - `CONTRIBUTING.md`
+  - `CODE_OF_CONDUCT.md`
+  - `SECURITY.md`
+  - `CHANGELOG.md`
+
+### Changed
+
+- Repository naming aligned to `sub2api-mobile` in public docs.