feat(voyage): add examples/observability/ Docker Compose stack — version-pinned per research/01

Step 14 of v4.1 — local-development observability stack with version-pinned
container images:
  - prom/prometheus:v3.0.1
  - prom/node-exporter:v1.10.2 (textfile collector enabled)
  - grafana/grafana:11.4.0
  - otel/opentelemetry-collector-contrib:0.115.0

Two complementary export paths from voyage hooks/scripts/otel-export.mjs:
  - VOYAGE_EXPORT_MODE=textfile → node-exporter textfile collector
  - VOYAGE_EXPORT_MODE=otlp     → otel-collector OTLP/HTTP receiver (:4318)
Both feed Prometheus → Grafana.

Files:
  examples/observability/docker-compose.yml
  examples/observability/otel-collector-config.yaml
  examples/observability/prometheus.yml
  examples/observability/grafana-datasource.yml
  examples/observability/README.md

Verified manifest expected_paths (5 files). docker compose config validation
runs in Step 16 with proper skip-pattern when docker is unavailable.

plugins/voyage/examples/observability/grafana-datasource.yml

@@ -0,0 +1,16 @@
+# Grafana datasource provisioning for voyage v4.1 observability stack.
+# Auto-loaded from /etc/grafana/provisioning/datasources/ on Grafana start.
+
+apiVersion: 1
+
+datasources:
+  - name: Prometheus
+    type: prometheus
+    access: proxy
+    url: http://prometheus:9090
+    isDefault: true
+    editable: true
+    jsonData:
+      timeInterval: 15s
+      httpMethod: POST
+      manageAlerts: false