feat: implement /assign GitHub Action

[shashank2k5]

Summary

This PR implements a GitHub Action that allows contributors to self-assign issues and pull requests using the /assign command. This automation reduces maintainer workload by enabling instant task ownership directly from the comment section.

Closes #

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Type of Change

• New feature
• Documentation
• Infrastructure / DevOps

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What Changed

• .github/workflows/assign.yml: Created a new workflow that parses /assign commands using actions/github-script. It handles self-assignment, mention-based assignment (/assign @user), duplicate prevention, and validation of GitHub usernames.
• CONTRIBUTING.md: Added a new "Self-Assigning Issues & PRs" section to guide new contributors on how to use the command and updated the PR process workflow.

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How to Test

1. Push this branch to your GitHub repository.
2. Navigate to an open Issue or PR on GitHub.
3. Post a comment with the text /assign.
4. Verify that the GitHub Actions bot assigns you to the item and replies with a confirmation message.
5. Try edge cases: Test /assign @username for another user or /assign on an item you are already assigned to.

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Checklist

• My code follows the project's coding style.
• I have updated documentation where necessary.
• No new console.log or debug statements left in the code.
• Breaking changes are documented in this PR description.

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Additional Context

The workflow uses github-token: ${{ secrets.GITHUB_TOKEN }} which has the necessary permissions defined in the YAML file (issues: write, pull-requests: write). It is designed to ignore bot comments to prevent infinite loops.