chore: Auto-update lockfiles in release-please PRs

[27Bslash6]

Problem

Release-please bumps version in pyproject.toml and rust/Cargo.toml but doesn't regenerate lockfiles. This leaves uv.lock and Cargo.lock with stale versions (e.g., 0.0.0 when manifests are at 0.1.0).

Current behavior

1. Release-please creates PR bumping pyproject.toml → 0.2.0
2. uv.lock still shows version = "0.1.0" for cachekit
3. Lockfile drift until someone runs uv sync locally

Proposed solution

Add a step to the release-please workflow that updates lockfiles after the PR is created:

- name: Update lockfiles
  if: steps.release.outputs.pr
  run: |
    git checkout ${{ steps.release.outputs.pr_branch }}
    uv lock
    cd rust && cargo generate-lockfile
    git add uv.lock Cargo.lock
    git commit -m "chore: sync lockfiles with version bump"
    git push

Alternatively, use a GitHub Action like peter-evans/create-pull-request to handle the commit cleanly.

Acceptance criteria

• When release-please bumps versions, lockfiles are updated in the same PR
• CI passes with synchronized lockfiles