docs: add CHANGELOG.md with historical release notes

Retroactively documents all releases from v0.1.0 to v0.4.0.
Format follows Keep a Changelog (https://keepachangelog.com/).

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

Author: CLCrawford-dev

CHANGELOG.md

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+# Changelog
+
+All notable changes to keep-protocol will be 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).
+
+## [Unreleased]
+
+## [0.4.0] — 2026-02-03
+
+### Added
+- `ensure_server()` auto-bootstrap function (KP-11)
+  - Checks if port 9009 is accepting connections
+  - Starts server via Docker if not running
+  - Falls back to `go install` if Docker unavailable
+  - Waits up to 30 seconds for server readiness
+- CI test workflow for Python and Go (KP-16)
+- Release workflow template for Linear issue structure
+- Mandatory staging workflow documentation
+
+### Fixed
+- pytest fixture error in test suite
+- go vet scope to exclude reference code
+
+### Documentation
+- Added `ensure_server()` usage to README
+- Refined virality plan for v0.3.0+
+- Sandbox testing step in release checklist
+
+## [0.3.1] — 2026-02-03
+
+### Fixed
+- Excluded `keep-protocol-clawhub/` from `go vet` (reference code, not buildable)
+- Updated release checklist with proper tag-first workflow
+
+## [0.3.0] — 2026-02-03
+
+### Added
+- **Discovery system** (KP-7)
+  - `discover("info")` — server version, agents online, uptime
+  - `discover("agents")` / `discover_agents()` — list connected agents
+  - `discover("stats")` — scar exchange counts, total packets
+- **Endpoint caching** — `~/.keep/endpoints.json`
+- **`from_cache()` client** — reconnect from cached endpoints
+- **Scar logging** — server tracks scar field exchanges
+- Discovery feature test suite (KP-10)
+
+### Changed
+- Wire protocol now uses length-prefixed framing (KP-1)
+  - `[4 bytes: uint32 big-endian length][N bytes: protobuf Packet]`
+  - Maximum payload: 65,536 bytes
+  - **Breaking:** Raw protobuf writes no longer accepted
+
+### Documentation
+- SKILL.md for ClawHub/AI agent discovery (KP-6)
+- AGENTS.md for AI coding assistant integration
+- ClawHub publishing guide with registry workaround
+- Routing examples and v0.2.0 protocol docs (KP-4)
+
+## [0.2.0] — 2026-02-03
+
+### Added
+- **Agent-to-agent routing** (KP-2)
+  - Server maintains identity-based routing table
+  - First signed packet's `src` field registers the connection
+  - `dst="bot:alice"` forwards to registered agent
+- **Persistent connections** with `listen()` in Python SDK (KP-3)
+- Length-prefixed wire framing (KP-1)
+
+### Changed
+- Wire format requires 4-byte length prefix (breaking change from v0.1.x)
+
+## [0.1.1] — 2026-02-02
+
+### Added
+- Multi-arch Docker image (amd64 + arm64)
+
+### Fixed
+- CI: Added `contents:read` permission to PyPI publish job
+- CI: Lowercase ghcr.io tags (Docker requires lowercase)
+
+## [0.1.0] — 2026-02-02
+
+### Added
+- Initial release
+- Go server with ed25519 signature verification
+- Python SDK (`KeepClient`)
+- Protobuf packet schema (`keep.proto`)
+- Docker image at `ghcr.io/clcrawford-dev/keep-server`
+- PyPI package `keep-protocol`
+
+### Protocol
+- Packets must be ed25519 signed (unsigned = silent drop)
+- Fields: sig, pk, typ, id, src, dst, body, fee, ttl, scar
+- Default port: 9009
+
+---
+
+[Unreleased]: https://github.com/CLCrawford-dev/keep-protocol/compare/v0.4.0...HEAD
+[0.4.0]: https://github.com/CLCrawford-dev/keep-protocol/compare/v0.3.1...v0.4.0
+[0.3.1]: https://github.com/CLCrawford-dev/keep-protocol/compare/v0.3.0...v0.3.1
+[0.3.0]: https://github.com/CLCrawford-dev/keep-protocol/compare/v0.1.1...v0.3.0
+[0.2.0]: https://github.com/CLCrawford-dev/keep-protocol/compare/v0.1.1...v0.2.0
+[0.1.1]: https://github.com/CLCrawford-dev/keep-protocol/compare/v0.1.0...v0.1.1
+[0.1.0]: https://github.com/CLCrawford-dev/keep-protocol/releases/tag/v0.1.0