chore(ci): remove obsolete Dependabot policy

[P4suta]

Summary

• Remove the unused Dependabot auto-merge policy and its test after the organization moved dependency updates to Renovate.
• Remove the dead CI step so contributors no longer maintain two automation models.

Verification

• actionlint .github/workflows/*.yml
• Repository pre-push dependency boundary

[coderabbitai]

Important

• 🔍 Trigger review

This repository does not receive automatic reviews because it has fewer than 10 stars.

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Advanced

Run ID: 6b49e98d-0381-4c2b-84ab-aabc4c89a559

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