test(voyage): add tests/integration/observability-compose.test.mjs — SC #16 skip-if-no-docker pattern

Step 16 of v4.1 — first test in tests/integration/, establishes the
skip-on-missing-tool pattern voyage will reuse for environment-dependent
integration tests. Two tests:
  1. compose config parses and contains expected services
  2. compose config pins required image versions

Both skip cleanly when 'docker info' fails (no Docker installed). On a
machine with Docker, both tests run docker compose config and assert the
4 services + 3 version pins are present.

Tests: 468 pass + 2 skipped (Docker not installed in dev env).

plugins/voyage/tests/integration/observability-compose.test.mjs

@@ -0,0 +1,59 @@
+// SC #16 — skip-if-no-docker compose-config validation.
+// First test in tests/integration/ — establishes the skip-on-missing-tool
+// pattern voyage uses for environment-dependent integration tests.
+
+import { test } from 'node:test';
+import { strict as assert } from 'node:assert';
+import { execFileSync, spawnSync } from 'node:child_process';
+import { fileURLToPath } from 'node:url';
+import { dirname, resolve } from 'node:path';
+
+const __dirname = dirname(fileURLToPath(import.meta.url));
+const REPO_ROOT = resolve(__dirname, '../..');
+const COMPOSE_FILE = resolve(REPO_ROOT, 'examples/observability/docker-compose.yml');
+
+const dockerAvailable = (() => {
+  try {
+    execFileSync('docker', ['info'], { stdio: 'ignore' });
+    return true;
+  } catch {
+    return false;
+  }
+})();
+
+test(
+  'compose config parses and contains expected services',
+  { skip: !dockerAvailable && 'Docker not installed' },
+  () => {
+    const r = spawnSync(
+      'docker',
+      ['compose', '-f', COMPOSE_FILE, 'config'],
+      { encoding: 'utf8' },
+    );
+    assert.equal(r.status, 0, `docker compose config exited ${r.status}: ${r.stderr}`);
+    assert.match(r.stdout, /otel-collector/, 'otel-collector service missing');
+    assert.match(r.stdout, /prometheus/, 'prometheus service missing');
+    assert.match(r.stdout, /grafana/, 'grafana service missing');
+    assert.match(r.stdout, /node-exporter/, 'node-exporter service missing');
+  },
+);
+
+test(
+  'compose config pins required image versions',
+  { skip: !dockerAvailable && 'Docker not installed' },
+  () => {
+    const r = spawnSync(
+      'docker',
+      ['compose', '-f', COMPOSE_FILE, 'config'],
+      { encoding: 'utf8' },
+    );
+    assert.equal(r.status, 0);
+    assert.match(r.stdout, /prom\/prometheus:v3\.0\.1/, 'prometheus pin missing');
+    assert.match(r.stdout, /grafana\/grafana:11\.4\.0/, 'grafana pin missing');
+    assert.match(
+      r.stdout,
+      /otel\/opentelemetry-collector-contrib:0\.115\.0/,
+      'otel-collector pin missing',
+    );
+  },
+);