📝 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>

documentation/API.md

@@ -0,0 +1,143 @@
+# API Endpoints
+
+REST API reference for `dance-lessons-coach`. Extracted from the original `AGENTS.md` (Tâche 6 restructure) for lazy-loading compatibility with Mistral Vibe.
+
+## Base URL
+
+```
+http://localhost:8080
+```
+
+## OpenAPI Documentation
+
+- **Swagger UI:** `http://localhost:8080/swagger/`
+- **OpenAPI Spec:** `http://localhost:8080/swagger/doc.json`
+
+The API provides interactive documentation using Swagger UI with complete OpenAPI 2.0 specification. All endpoints, request/response models, and validation rules are documented using a **hierarchical tagging system**.
+
+**Features:**
+
+- Interactive API exploration with hierarchical organization
+- Try-it-out functionality for all endpoints
+- Model schemas with examples
+- Response examples with validation rules
+- Hierarchical tag structure for better navigation
+
+**Generation:** Documentation is auto-generated from code annotations using [swaggo/swag](https://github.com/swaggo/swag) with the command:
+
+```bash
+go generate ./pkg/server/
+```
+
+**Tag Organization:**
+
+- `API/v1/Greeting` — Version 1 greeting endpoints
+- `API/v2/Greeting` — Version 2 greeting endpoints
+- `System/Health` — Health and readiness endpoints
+
+**Hierarchical Benefits:**
+
+- Clear separation between API domains (API vs System)
+- Version organization within each domain
+- Natural hierarchy in Swagger UI
+- Scalable for future API growth
+
+**Embedded Documentation:** The OpenAPI spec is embedded in the binary using Go's `//go:embed` directive for single-binary deployment.
+
+---
+
+## Health Check
+
+```http
+GET /api/health
+```
+
+**Response:**
+
+```json
+{"status":"healthy"}
+```
+
+## Readiness Check
+
+```http
+GET /api/ready
+```
+
+**Responses:**
+
+- Normal operation: `{"ready":true}` (HTTP 200)
+- During shutdown: `{"ready":false}` (HTTP 503 Service Unavailable)
+
+**Purpose:** Indicates whether the server is ready to accept new requests. Returns false during graceful shutdown to allow existing requests to complete while preventing new ones.
+
+## Greet Service v1
+
+```http
+GET /api/v1/greet/
+GET /api/v1/greet/{name}
+```
+
+**Examples:**
+
+```bash
+# Default greeting
+curl http://localhost:8080/api/v1/greet/
+# Response: {"message":"Hello world!"}
+
+# Personalized greeting
+curl http://localhost:8080/api/v1/greet/John
+# Response: {"message":"Hello John!"}
+
+# Another example
+curl http://localhost:8080/api/v1/greet/Alice
+# Response: {"message":"Hello Alice!"}
+```
+
+## Greet Service v2 (Feature-flagged)
+
+```http
+POST /api/v2/greet
+```
+
+**Request Body:**
+
+```json
+{
+  "name": "John"
+}
+```
+
+**Examples:**
+
+```bash
+# Valid request
+curl -X POST http://localhost:8080/api/v2/greet \
+  -H "Content-Type: application/json" \
+  -d '{"name":"John"}'
+# Response: {"message":"Hello my friend John!"}
+
+# Empty name (valid, returns default)
+curl -X POST http://localhost:8080/api/v2/greet \
+  -H "Content-Type: application/json" \
+  -d '{"name":""}'
+# Response: {"message":"Hello my friend!"}
+
+# Missing name field (valid, returns default)
+curl -X POST http://localhost:8080/api/v2/greet \
+  -H "Content-Type: application/json" \
+  -d '{}'
+# Response: {"message":"Hello my friend!"}
+
+# Name too long (validation error)
+curl -X POST http://localhost:8080/api/v2/greet \
+  -H "Content-Type: application/json" \
+  -d '{"name":"ThisNameIsWayTooLongAndShouldFailValidationBecauseItExceedsTheMaximumAllowedLengthOf100Characters!!!!"}'
+# Response: {"error":"validation_failed","message":"Invalid request data","details":[{"message":"Name failed validation for 'max' (parameter: 100)"}]}
+```
+
+**Validation Rules:**
+
+- `name`: Maximum length 100 characters (optional field)
+
+**Feature Flag:** Enable with `DLC_API_V2_ENABLED=true` or in config file with `api.v2_enabled: true`.