chore(mcp): use XCE_API_KEY env var in xce-scheduler MCP config

Reference the operator key via ${XCE_API_KEY} instead of a hardcoded value —
apiserver restarts clear in-memory keys, so a committed key goes stale
silently and is a credential risk. Export XCE_API_KEY before starting Claude
Code to use the scheduler-agent MCP server.

Co-Authored-By: Claude <noreply@anthropic.com>

.mcp.json

@@ -1,3 +1,13 @@
 {
-  "mcpServers": {}
+  "mcpServers": {
+    "xce-scheduler": {
+      "type": "stdio",
+      "command": "/tmp/xce-mcp",
+      "args": [],
+      "env": {
+        "XCE_API_URL": "http://172.31.1.30:8080",
+        "XCE_API_KEY": "${XCE_API_KEY}"
+      }
+    }
+  }
 }