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/prometheus.yml

@@ -0,0 +1,31 @@
+# Prometheus config for voyage v4.1 observability stack.
+# Two scrape targets:
+#   1. node-exporter — picks up voyage.prom files written by hooks/scripts/otel-export.mjs
+#      when VOYAGE_EXPORT_MODE=textfile (default location: ./voyage-textfile/)
+#   2. otel-collector — exposes voyage metrics from OTLP push when VOYAGE_EXPORT_MODE=otlp
+
+global:
+  scrape_interval: 15s
+  evaluation_interval: 15s
+  external_labels:
+    monitor: voyage-local
+
+scrape_configs:
+  # Path A: voyage textfile mode → node-exporter textfile collector
+  - job_name: voyage-textfile
+    static_configs:
+      - targets: ["node-exporter:9100"]
+        labels:
+          voyage_export_mode: textfile
+
+  # Path B: voyage OTLP mode → otel-collector prometheus exporter
+  - job_name: voyage-otlp
+    static_configs:
+      - targets: ["otel-collector:8889"]
+        labels:
+          voyage_export_mode: otlp
+
+  # Self-scrape so Prometheus shows its own up=1 in dashboards.
+  - job_name: prometheus
+    static_configs:
+      - targets: ["localhost:9090"]