📝 docs(restructure): split AGENTS.md into focused guides (Tâche 6 Phase B)

Création de 9 fichiers neufs pour décharger AGENTS.md (1296 lignes →
~130) en documents lazy-loadables, compatibles avec la limite de
contexte 128k de Mistral Vibe (cf. ARCODANGE migration Phase 1,
Tâche 6 du curriculum).

Sept guides ciblés sous documentation/ :
- HISTORY.md            : phases historiques 1-9 du développement
- CLI.md                : commandes CLI, server lifecycle, config DLC_*
- API.md                : endpoints REST, OpenAPI, Greet v1/v2
- OBSERVABILITY.md      : OpenTelemetry + Jaeger, sampler types, test
- TROUBLESHOOTING.md    : issues connues + pointeurs vers guides spé
- CODE_EXAMPLES.md      : snippets endpoint/logging/context, pointeurs ADR
- ROADMAP.md            : potential features, architectural improvements

Deux fichiers racine :
- CHANGELOG.md          : user-facing, format Keep a Changelog
- AGENT_CHANGELOG.md    : décisions structurantes des agents AI
                          (référencé par AGENTS.md, n'existait pas)

Le contenu est extrait fidèlement d'AGENTS.md sans réinterprétation.
Phase C (réécriture AGENTS.md court) suit dans le commit suivant.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

CHANGELOG.md

@@ -0,0 +1,57 @@
+# Changelog
+
+Notable user-facing changes to `dance-lessons-coach`. Format inspired by [Keep a Changelog](https://keepachangelog.com/), versioning follows [Semantic Versioning 2.0.0](https://semver.org/) (see [`documentation/version-management-guide.md`](documentation/version-management-guide.md)).
+
+The historical phases of foundational development (Phase 1 to Phase 9) are documented in [`documentation/HISTORY.md`](documentation/HISTORY.md).
+
+## [Unreleased]
+
+### Added
+
+_(items pending release; move to a versioned section when tagged)_
+
+### Changed
+
+### Fixed
+
+---
+
+## 2026-04-05 — Architecture Documentation
+
+- ✅ Added comprehensive ADR directory with 9 decision records
+- ✅ Enhanced Zerolog vs Zap analysis in logging ADR
+- ✅ Updated `README.md` and `AGENTS.md` with ADR references
+- ✅ Documented hybrid testing approach
+- ✅ Added BDD testing decision record
+
+## 2026-04-04 — Observability & Testing
+
+- ✅ OpenTelemetry integration with Jaeger
+- ✅ Middleware-only tracing approach
+- ✅ Comprehensive telemetry configuration
+- ✅ BDD testing framework setup
+- ✅ Hybrid testing strategy documentation
+
+## 2026-04-03 — Production Readiness
+
+- ✅ Graceful shutdown with readiness endpoints
+- ✅ Configuration management with Viper
+- ✅ JSON logging configuration
+- ✅ File output logging support
+- ✅ Comprehensive error handling
+
+## 2026-04-02 — Web API Foundation
+
+- ✅ Chi router integration
+- ✅ Versioned API endpoints (`/api/v1`)
+- ✅ Health and readiness endpoints
+- ✅ JSON responses with proper headers
+- ✅ Interface-based design patterns
+
+## 2026-04-01 — Project Foundation
+
+- ✅ Go 1.26.1 environment setup
+- ✅ Project structure with `cmd/` and `pkg/`
+- ✅ Core Greet service implementation
+- ✅ CLI interface
+- ✅ Unit tests with table-driven approach