chore(renovate): group mise.toml updates into a single PR

Reduce Renovate PR load by grouping all mise tool updates (mise.toml)
into one "mise tools" PR, mirroring the file-scoped grouping already used
for ui/** and the e2e docker-compose file.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: nlevee

renovate.json

@@ -60,6 +60,18 @@
         "ui"
       ]
     },
+    {
+      "matchFileNames": [
+        "mise.toml"
+      ],
+      "separateMajorMinor": false,
+      "separateMinorPatch": false,
+      "groupName": "mise tools",
+      "groupSlug": "mise-tools",
+      "addLabels": [
+        "mise"
+      ]
+    },
     {
       "groupName": "go github sdk",
       "groupSlug": "go-github",