Add Dependabot configuration

[ScottMorris]

Context

This repo has no Dependabot configuration. Dependencies (npm via pnpm) are updated manually.

Proposal

Add .github/dependabot.yml:

version: 2
updates:
  - package-ecosystem: "npm"
    directory: "/"
    schedule:
      interval: "weekly"

  - package-ecosystem: "github-actions"
    directory: "/"
    schedule:
      interval: "monthly"

Acceptance Criteria

• dependabot.yml committed
• First batch of Dependabot PRs reviewed and merged/closed