feat(voyage): wire Stop event to otel-export.mjs in hooks.json

Step 13 of v4.1 — adds Stop hook entry pointing to
hooks/scripts/otel-export.mjs (added in Step 12 / commit c5fb745).
Mounts the orchestrator on Claude Code's Stop event so OTel/Prometheus
export runs at session-end when VOYAGE_EXPORT_MODE is set.

HIGH-risk-mitigering: tests/hooks/hooks-json-stop-wired.test.mjs
asserter at Stop-key finnes, refererer otel-export.mjs, bruker
\${CLAUDE_PLUGIN_ROOT}-substitusjon, og har type:command.

Tests: 464 → 468 (4 new). All green.

plugins/voyage/hooks/hooks.json

@@ -60,6 +60,16 @@
           }
         ]
       }
+    ],
+    "Stop": [
+      {
+        "hooks": [
+          {
+            "type": "command",
+            "command": "node ${CLAUDE_PLUGIN_ROOT}/hooks/scripts/otel-export.mjs"
+          }
+        ]
+      }
     ]
   }
 }