Add Dependabot config for monthly grouped Go modules and GitHub Actions updates

[Copilot]

Adds repository-level Dependabot automation to keep dependencies current with low PR noise. Go module updates are grouped into one monthly PR, while GitHub Actions updates are grouped into a separate monthly PR.

• What changed

  • Added .github/dependabot.yml with two update ecosystems:
    • gomod at repository root (/)
    • github-actions at repository root (/)
  • Configured both to run on a monthly schedule.
  • Grouped each ecosystem into a single PR stream to avoid fragmented update PRs.

• Dependabot configuration

  version: 2
  updates:
    - package-ecosystem: gomod
      directory: "/"
      schedule:
        interval: monthly
      groups:
        go-dependencies:
          patterns:
            - "*"
  
    - package-ecosystem: github-actions
      directory: "/"
      schedule:
        interval: monthly
      groups:
        github-actions-dependencies:
          patterns:
            - "*"