Automate dependency and Actions updates, and pin what CI runs

[SociableSteve]

Problem

Dependency maintenance is half automated, and the half that is missing is the one that keeps the
repo current rather than merely unbreached.

Dependabot security updates are enabled on the repository, so a published advisory against
something in package-lock.json raises a pull request. Nothing covers the rest:

• No version updates. There is no .github/dependabot.yml, so ordinary upgrades happen only
  when somebody thinks to run them. A dependency tree that only ever moves under advisory pressure
  drifts until an upgrade is a project rather than a pull request, and Caroline pins itself to Node
  24 and a specific MCP protocol revision, which makes drift consequential.
• The Actions are not covered at all. The CI workflow uses actions/checkout@v4 and
  actions/setup-node@v4, floating tags that move under the workflow. Security updates do not
  watch Actions, so nothing raises a pull request when one of them changes or is deprecated, and
  nothing pins what is actually run.

This is a repository that handles Google OAuth refresh tokens, GitHub tokens and LLM provider API
keys, and whose CI runs on every pull request.

Expected outcome

Staying current is routine and visible rather than remembered.

• Ordinary dependency upgrades arrive as pull requests on a schedule, for the npm manifest and for
  the GitHub Actions, without anybody asking for them.
• The volume is manageable rather than noise. A wall of individual patch bumps every morning gets
  ignored, and an ignored update stream is the same as no update stream.
• What CI actually runs is pinned to something immutable rather than to a tag that can be moved,
  and updating that pin is an ordinary reviewed change.
• The existing hardening in the workflow survives the change: least-privilege permissions and
  persist-credentials: false, both of which are there deliberately and have their reason written
  down.
• CI stays the gate. An upgrade is merged because lint, typecheck, the suite and both builds
  passed, not because a bot proposed it.