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 to automate and standardize GitHub Actions dependency updates for the thold plugin repository, reducing update noise while keeping workflows current.

Changes:

• Introduces .github/dependabot.yml with weekly scheduled GitHub Actions updates targeting the develop branch.
• Enables grouped updates (single group matching all actions), applies standard labels, and standardizes commit message prefix/scope.
• Adds a 7-day cooldown window to avoid immediate PRs for newly released versions.

[somethingwithproof]

Rebased onto current develop (branch was 11 commits behind). No Copilot review threads to address.

The Integration Test failures are unrelated to this PR's diff (adds .github/dependabot.yml only, no workflow changes). Root cause: apt-get install ... libapache2-mod-php{8.1,8.2,8.4} fails with "Unable to locate package" on the ubuntu-latest runner image, since the default apt repos only ship libapache2-mod-php8.3. This also fails on the latest push to develop (run 26199721468, 2026-05-21), so it's a pre-existing repo-wide CI infra issue, not something introduced here. Not fixing it as part of this PR.