ci: add auto-assign workflow

[gvieira18]

Summary

• Add auto-assign workflow that assigns PR authors automatically on open/reopen
• Uses shared he4rt/.github/actions/auto-assign composite action pinned by SHA
• Standardize dependabot.yml with github-actions ecosystem monitoring (weekly, 7-day cooldown, ci commit prefix)

Test plan

• Open a test PR to verify the author is auto-assigned
• Verify Dependabot picks up github-actions updates on next weekly scan

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Repository YAML (base), Central YAML (inherited)

Review profile: CHILL

Plan: Pro

Run ID: e311e70f-0be2-4752-a690-da5e89d5a4f2

📥 Commits

Reviewing files that changed from the base of the PR and between 6b75912 and eac4899.

📒 Files selected for processing (2)

• .github/dependabot.yml
• .github/workflows/auto-assign.yml

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📝 Walkthrough

Walkthrough

PR adds two CI configuration updates: refines dependabot scheduling for GitHub Actions with weekly interval, 7-day cooldown, and ci-prefixed commit messages; adds auto-assign workflow triggering on opened/reopened pull requests to the 4.x branch, with minimal permissions and concurrency deduplication.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title 'ci: add auto-assign workflow' accurately summarizes the main change: adding a new GitHub Actions workflow for auto-assigning PR authors.
Description check	✅ Passed	The description directly relates to the changeset, detailing both the auto-assign workflow addition and the dependabot.yml standardization, matching the file modifications.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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