chore: fix dependabot PR settings — remove self-assignment, add auto squash-and-merge

[rania-run]

Problem

Dependabot PRs are currently assigning rania-run as an assignee alongside dependabot. This is noisy and incorrect — rania-run should only appear as a reviewer, not an assignee.

Additionally, once a dependabot PR is approved, it should automatically squash-and-merge without manual intervention.

Tasks

• Update .github/dependabot.yml to remove rania-run from assignees
• Add rania-run as a reviewer in the dependabot config instead
• Configure auto squash-and-merge for dependabot PRs — via one of:
  • GitHub repo settings: enable Auto-merge + add a branch ruleset/protection requiring approval
  • A .github/workflows/dependabot-automerge.yml workflow that calls gh pr merge --squash --auto once the PR is approved and checks pass

Expected Result

• Dependabot PRs: assignee = dependabot[bot] only
• Dependabot PRs: reviewer = rania-run
• Dependabot PRs: auto squash-and-merge after approval + passing checks