📝 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/CODE_EXAMPLES.md

@@ -0,0 +1,59 @@
+# Code Examples
+
+Snippets and patterns used across the `dance-lessons-coach` codebase. Extracted from the original `AGENTS.md` (Tâche 6 restructure).
+
+## Adding a New API Endpoint
+
+```go
+// 1. Add to interface
+func (h *apiV1GreetHandler) RegisterRoutes(router chi.Router) {
+    router.Get("/", h.handleGreetQuery)
+    router.Get("/{name}", h.handleGreetPath)
+    router.Post("/custom", h.handleCustomGreet) // New endpoint
+}
+
+// 2. Implement handler
+func (h *apiV1GreetHandler) handleCustomGreet(w http.ResponseWriter, r *http.Request) {
+    // Parse request
+    // Call service
+    // Return JSON response
+}
+```
+
+## Logging with Zerolog
+
+```go
+// Trace level logging
+log.Trace().Ctx(ctx).Str("key", "value").Msg("message")
+
+// Info level
+log.Info().Msg("Important event")
+
+// Error level
+log.Error().Err(err).Msg("Error occurred")
+```
+
+For the full logging strategy (when to use Trace vs Info, performance considerations), see [ADR-0003 — Zerolog Logging](../adr/0003-zerolog-logging.md).
+
+## Using `context.Context`
+
+```go
+// Pass context through calls
+func handler(w http.ResponseWriter, r *http.Request) {
+    result := service.Greet(r.Context(), "John")
+    // ...
+}
+
+// Create context with values
+ctx := context.WithValue(r.Context(), "key", "value")
+
+// Create context with timeout
+ctx, cancel := context.WithTimeout(r.Context(), 5*time.Second)
+defer cancel()
+```
+
+For the rationale behind context-aware services, see [ADR-0004 — Interface-Based Design](../adr/0004-interface-based-design.md).
+
+## Best Practices Reminders
+
+For higher-level guidance on code organization, error handling, performance, and testing, see [`AGENT_USAGE_GUIDE.md`](AGENT_USAGE_GUIDE.md#best-practices) section "Best Practices".