feat(workflow): add merged PR discord reminder for GSSoC labels

[Harxhit]

Summary

Adds an automated GitHub workflow that posts a reminder comment when a pull request is merged. The workflow congratulates contributors and reminds them to contact the maintainers on Discord for GSSoC label assignment and contributor recognition. It also ignores selected maintainer accounts to avoid unnecessary notifications.

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Type of Change

• New feature
• Bug fix
• Refactor (no functional change)
• UI / Design change
• Tests only
• Documentation
• Infrastructure / DevOps
• Security

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What Changed

• Added a new GitHub Actions workflow to trigger on merged pull requests.
• Added discordPinReminder.js script to post automated reminder comments on merged PRs.
• Added ignore-user handling to skip reminders for selected maintainer accounts.

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How to Test

1. Open a test pull request from a non-ignored contributor account.
2. Merge the pull request and verify the workflow runs successfully.
3. Confirm a congratulatory comment is posted with Discord/GSSoC label instructions.

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Checklist

• My code follows the project's coding style (pnpm -r run lint passes).
• TypeScript compiles without errors (pnpm -r run typecheck).
• I have added or updated tests for the changes I made.
• All tests pass locally (pnpm -r run test).
• 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

This workflow only triggers for newly merged pull requests and does not process previously merged PRs. Manual triggering via workflow_dispatch remains available for testing and maintenance.

[Harxhit]

@ShantKhatri Please look into this.