chore: switch Renovate lock-file maintenance & dev-deps to PR automerge (#867)

`lockFileMaintenance` and the "major/minor dev dependencies" rule used
`automergeType: "branch"`, which never opens a PR and relies on branch
automerge merging directly into `master` once branch status checks pass.
But CI only runs on `pull_request` and pushes to `master`, so no checks
ever run on `renovate/*` branches and the branch can never go green. The
update (including `uv.lock`) just piled up on the branch with no PR and
no merge.

Switch both rules to `automergeType: "pr"` so Renovate opens a PR (which
triggers the existing pull_request CI) and auto-merges it once green,
consistent with every other update in this repo.

Co-authored-by: Claude <noreply@anthropic.com>