chore: add Dependabot config for automated dependency updates

[PaulGrimshaw]

Context

No automated dependency update mechanism exists. Stale dependencies are a maintenance burden and a security risk.

Change required

Add .github/dependabot.yml:

version: 2
updates:
  - package-ecosystem: "npm"
    directory: "/"
    schedule:
      interval: "weekly"
    groups:
      dev-dependencies:
        dependency-type: "development"
  - package-ecosystem: "github-actions"
    directory: "/"
    schedule:
      interval: "weekly"

Using groups keeps the PR noise manageable.