[2.6] Add Dependabot auto-merge workflow

[vbonk]

What needs to be done

Create .github/workflows/dependabot-auto-merge.yml. Auto-merge minor/patch Dependabot PRs that pass CI. Only for dependabot[bot] author.

Acceptance Criteria

• Workflow exists
• Only triggers for dependabot[bot]
• Only minor/patch versions
• Requires CI passing

[vbonk]

Completed in Phase 2 PR #86.