chore(renovate): rebase PRs only when conflicted

Renovate's default `rebaseWhen: auto` rebases every open PR whenever the
base branch moves if the repository requires branches to be up to date.
Each rebase re-triggers the full CI matrix, so a single merge to `main`
fans out into a CI run per open dependency PR.

Set `rebaseWhen: conflicted` so Renovate only rebases a PR when it can
no longer merge cleanly. Automerge still runs on the PR's own successful
checks.

Assisted-by: Claude Code:claude-opus-5

Author: JP-Ellis

.github/renovate.json

@@ -9,6 +9,7 @@
   },
   "prHourlyLimit": 0,
   "prConcurrentLimit": 0,
+  "rebaseWhen": "conflicted",
   "automerge": true,
   "packageRules": [
     {