chore: add enterprise Dependabot configuration

[somethingwithproof]

Adds grouped, cooled-down Dependabot updates with standard ecosystem labels and commit prefixes.

[Copilot]

Pull request overview

Adds a Dependabot configuration intended to standardize and group GitHub Actions dependency update PRs (labels + commit message prefixing) and reduce update noise.

Changes:

• Configure Dependabot for the github-actions ecosystem on a weekly schedule targeting develop.
• Group all GitHub Actions updates into a single group and apply standard labels/commit message formatting.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[somethingwithproof]

@copilot Fix the code for all comments in this review thread.

When a review comment includes a suggested change, apply the suggestion exactly.

Do not make changes beyond what is described in the linked review thread.

[somethingwithproof]

cooldown is a real, supported Dependabot version-updates option (see https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file#cooldown) using the default-days key exactly as configured here, so Copilot's claim it's unsupported doesn't hold; leaving it as-is. The github-actions label finding was correct and is already fixed by 736cbfe (dropped from labels:, since only dependencies exists on this repo). Resolving both threads.