chore: remove dependabot config from sandbox (#235)

## Summary
- Delete `.github/dependabot.yml` so Dependabot stops opening
version-update PRs in this sandbox repo.

This repo only exists for CI testing, so we don't need automated
dependency PRs at all. Security updates can still be enabled via repo
Settings → Code security if desired (independent of this file).

Supersedes #234.

## Test plan
- [ ] No new Dependabot PRs after merge

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Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>