Update the self-hosted Renovate runner monthly

Upstream Renovate releases several times a day, which produced about
45 runner bump commits per month. Staying on the very latest version
brings little benefit, so these updates now happen once a month.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

.github/renovate.json

@@ -97,7 +97,7 @@
       "automergeType": "branch"
     },
     {
-      "description": "Automerge updates of the self-hosted Renovate runner (via branch push - no PR)",
+      "description": "Automerge updates of the self-hosted Renovate runner (via branch push - no PR). Monthly, since upstream releases several times a day and staying on the very latest version brings little benefit.",
       "matchManagers": [
         "custom.regex"
       ],
@@ -105,6 +105,9 @@
         "matrix-renovate-runner"
       ],
       "allowedVersions": "!/^44\\.64\\.0$/",
+      "schedule": [
+        "* 0-5 1 * *"
+      ],
       "automerge": true,
       "automergeType": "branch"
     },