📝 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/OBSERVABILITY.md

@@ -0,0 +1,94 @@
+# Observability — OpenTelemetry & Jaeger Integration
+
+Tracing setup for `dance-lessons-coach`. Extracted from the original `AGENTS.md` (Tâche 6 restructure) for lazy-loading compatibility with Mistral Vibe.
+
+The application supports OpenTelemetry for distributed tracing with Jaeger compatibility.
+
+## Configuration
+
+Enable OpenTelemetry in your `config.yaml`:
+
+```yaml
+telemetry:
+  enabled: true
+  otlp_endpoint: "localhost:4317"
+  service_name: "dance-lessons-coach"
+  insecure: true
+  sampler:
+    type: "parentbased_always_on"
+    ratio: 1.0
+```
+
+Or via environment variables:
+
+```bash
+export DLC_TELEMETRY_ENABLED=true
+export DLC_TELEMETRY_OTLP_ENDPOINT="localhost:4317"
+export DLC_TELEMETRY_SERVICE_NAME="dance-lessons-coach"
+export DLC_TELEMETRY_INSECURE=true
+export DLC_TELEMETRY_SAMPLER_TYPE="parentbased_always_on"
+export DLC_TELEMETRY_SAMPLER_RATIO=1.0
+```
+
+## Testing with Jaeger
+
+**1. Start Jaeger in Docker:**
+
+```bash
+docker run -d --name jaeger \
+  -e COLLECTOR_OTLP_ENABLED=true \
+  -p 16686:16686 \
+  -p 4317:4317 \
+  jaegertracing/all-in-one:latest
+```
+
+**2. Start the server with OpenTelemetry enabled:**
+
+```bash
+# Using config file
+./scripts/start-server.sh start
+
+# Or with environment variables
+DLC_TELEMETRY_ENABLED=true ./scripts/start-server.sh start
+```
+
+**3. Make API requests:**
+
+```bash
+curl http://localhost:8080/api/v1/greet/John
+```
+
+**4. View traces in Jaeger UI:**
+
+Open http://localhost:16686 and select the `dance-lessons-coach` service.
+
+## Sampler Types
+
+| Sampler | Behavior |
+|---|---|
+| `always_on` | Sample all traces |
+| `always_off` | Sample no traces |
+| `traceidratio` | Sample based on trace ID ratio |
+| `parentbased_always_on` | Sample based on parent span (always on) |
+| `parentbased_always_off` | Sample based on parent span (always off) |
+| `parentbased_traceidratio` | Sample based on parent span with ratio |
+
+## Testing Script
+
+A convenience script is provided:
+
+```bash
+./scripts/test-opentelemetry.sh
+```
+
+This script:
+
+1. Starts Jaeger container
+2. Starts the server with OpenTelemetry
+3. Makes test API calls
+4. Shows Jaeger UI URL
+5. Cleans up on exit
+
+## ADR Reference
+
+See [ADR-0007 — OpenTelemetry Integration](../adr/0007-opentelemetry-integration.md) for the full architectural decision and rationale (middleware-only approach, sampling strategy, OTLP/gRPC choice).