ci: add testing, linting, and Dependabot auto-merge

[moyom96]

Context

The repository had no CI. There was nothing enforcing tests or style on pull requests, and Dependabot updates required fully manual review and merge. This adds automated testing, linting, and hands-off merging for low-risk dependency bumps.

What Changed

• .github/workflows/ci.yml runs on pushes to main and all pull requests, with two parallel jobs pinned to Ruby 3.3.6 (matching mise.toml):
  • lint → bundle exec standardrb
  • test → bundle exec rspec
  • Uses bundler-cache: true and cancels superseded in-progress runs per ref.
• .github/dependabot.yml schedules weekly version updates for bundler and github-actions, labeled dependencies.
• .github/workflows/dependabot-auto-merge.yml reads update metadata via dependabot/fetch-metadata and enables auto-merge (squash) only for semver-minor and semver-patch bumps. Major updates are left for manual review.

How to Test

• Open this PR and confirm the lint and test checks appear and run.
• After merge, confirm Dependabot opens its next round of PRs against the new config.
• On a minor/patch Dependabot PR, confirm the auto-merge workflow runs and marks the PR for auto-merge; on a major bump, confirm it does not.

Deployment Tasks

Auto-merge will not function on code alone. In repository settings:

• Enable Settings → General → Allow auto-merge.
• Add a branch protection rule on main requiring the lint and test status checks to pass. Without this, an auto-merged PR could merge before CI is green.

Notes

• CI on this branch will be red until the separate PR fixing the flay_collector_spec require and the existing Standard violations lands on main. No source files were touched here, since those fixes are handled separately.