chore: add CHANGELOG.md

CHANGELOG.md

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+# Changelog
+
+All notable changes to this project are documented in this file.
+
+The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
+and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
+
+Commit-message convention: [Conventional Commits](https://www.conventionalcommits.org/en/v1.0.0/)
+(`feat:`, `fix:`, `chore:`, `docs:`, `refactor:`, `test:`, `ci:`).
+
+## [Unreleased]
+
+### Added
+
+### Changed
+
+### Fixed
+
+## [0.1.0] - 2026-04-14
+
+Proof-of-concept distributed identity and trust protocol for AI agent networks.
+
+### Added
+
+- ECDSA P-256 identity: cryptographic keypairs with NodeID derivation
+- EMA trust scoring: decay-based trust with configurable thresholds (0.7 / 0.4 / 0.2)
+- 3-layer coherence: hash chain, schema, and temporal monotonicity validation
+- Hash-chained ledger: append-only event log with canonical JSON serialization
+- HTTP protocol: heartbeat, peer discovery, challenge, join endpoints
+- Standalone binary via `go install github.com/cogos-dev/constellation/cmd/constellation@v0.1.0`
+
+[Unreleased]: https://github.com/cogos-dev/constellation/compare/v0.1.0...HEAD
+[0.1.0]: https://github.com/cogos-dev/constellation/releases/tag/v0.1.0