Add Dependabot auto-merge workflow

[adamtheturtle]

Adds the Dependabot auto-merge workflow already used across the other repositories in Coding-Project-TODOs, so Dependabot PRs merge themselves once required checks pass.

---

Note

Low Risk
CI-only automation scoped to Dependabot PRs; no application or runtime code changes.

Overview
Adds a new GitHub Actions workflow Dependabot auto-merge that runs on every pull_request event.

When the PR author is dependabot[bot], the job enables auto-merge via gh pr merge --auto --merge, so the PR merges automatically once branch protection and required checks pass. The workflow grants contents: write and pull-requests: write and uses GITHUB_TOKEN for the CLI.

^{Reviewed by Cursor Bugbot for commit 389ecb7. Bugbot is set up for automated code reviews on this repo. Configure here.}