docs: add CHANGELOG.md (Keep-a-Changelog format)

Author: SIN-Agent

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/1.0.0/),
+and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
+
+## [Unreleased]
+
+### Added
+- Initial SIN-Code-Bundle integration (ceo-audit workflow v3)
+- OpenCode MCP server registration under `OpenSIN-Code/SIN-Code-FireworksAI-OpenCode-Config`
+- Repository-level `SIN_GITHUB_FALLBACK_TOKEN` secret for the App commenter fallback
+- Fireworks AI pool config for OpenCode CLI: 12 models at temperature 0
+- Base URL `https://sinatorpool-router.delqhi.com/inference/v1` with auto-failover
+- One-line install via `opencode-config-install.sh` / `opencode-config-repair.sh`
+- `config.example.json`, `INSTALL.md`, `requirements.txt`, and `tests/` for validation
+
+### Security
+- All commits verified via `git-immortal-commit` (annotated tags)
+- No real API keys committed — `config.example.json` ships with placeholders only