feat: add PR lifecycle workflow configuration

[0xisk]

Types of changes

What types of changes does your code introduce to OpenZeppelin Midnight Contracts?
Put an x in the boxes that apply

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update (if none of the other choices apply)

Fixes #70

PR Checklist

• I have read the Contributing Guide
• I have added tests that prove my fix is effective or that my feature works
• I have added documentation of new methods and any new behavior or changes to existing behavior
• CI Workflows Are Passing

Further comments

If this is a relatively large or complex change, kick off the discussion by explaining why you chose the solution you did and what alternatives you considered, etc...

Summary by CodeRabbit

• Chores
  • Added automated PR lifecycle management workflows to improve development processes.

[coderabbitai]

Walkthrough

A new GitHub Actions workflow file is added to automate pull request lifecycle management. The workflow triggers on multiple PR events (opened, reopened, converted_to_draft, ready_for_review, closed) and delegates to a reusable workflow hosted in an external repository.

Changes

Cohort / File(s)	Summary
GitHub Actions Configuration .github/workflows/pr-lifecycle.yml	New PR lifecycle workflow that triggers on multiple pull request events and invokes a reusable workflow from an external repository with project token secret forwarding.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~3 minutes

Suggested reviewers

• andrew-fleming

Poem

🐰 A workflow born, so sleek and lean,
PR events keep things clean,
Opens, reopens, drafts and more,
Automation through the door! ✨

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title 'feat: add PR lifecycle workflow configuration' clearly and specifically describes the main change: adding a new PR lifecycle workflow configuration file to the repository.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.

_{✏️ Tip: You can configure your own custom pre-merge checks in the settings.}

✨ Finishing Touches

• 📝 Generate docstrings (stacked PR)
• 📝 Generate docstrings (commit on current branch)

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch feat/pr-lifecycle

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