chore(renovate): automerge lock-file maintenance

lockFileMaintenance was enabled but not automerging, so refreshes opened PRs that
then aged - producing exactly the lock lag the setting exists to prevent. Lock lag
is the staleness that actually recurs in this fleet: a caret requirement that can
reach a newer version while the committed lock sits behind it. rangeStrategy does
not fix that; automerged lock maintenance does.

Enabled only because this repo has an MSRV job, per ADR-0018: automerge is gated on
the MSRV promise being protected by CI rather than by a reviewer noticing. Renovate
also waits for checks to pass before automerging, so a bump that raises the minimum
Rust version fails the MSRV job and cannot land. Repos without an MSRV job were
deliberately skipped in this sweep.

chore, not fix, so this does not cut a release: no crate code changed.

Verified: renovate.json parses as JSON and lockFileMaintenance.automerge is true.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: h4x0r

renovate.json

@@ -43,6 +43,7 @@
   ],
   "lockFileMaintenance": {
     "enabled": true,
+    "automerge": true,
     "schedule": [
       "before 5am on the first day of the month"
     ]