Add renovate.json

Closes #2244 and #2245. Prior-art hits in a different language than the file
being written are now labelled and explained rather than surfaced bare, and
get_task records a pull so auto-inject's pull-through stops reading near-zero for
the kind it mostly surfaces.

No migration, no plugin manifest bump — server-side only.

renovate.json

@@ -0,0 +1,6 @@
+{
+  "$schema": "https://docs.renovatebot.com/renovate-schema.json",
+  "extends": [
+    "config:recommended"
+  ]
+}