Auto-welcome First-Time Contributors

# Auto-welcome First-Time Contributors

## Goal
Create a GitHub Action that welcomes first-time contributors who open PRs adding a single entry to `Contributors.md`.

## Requirements

**Trigger**
- pull_request (opened, synchronize)

**Validate**
- Only `Contributors.md` is changed
- Exactly one new entry added
- Format:
* [Name](https://github.com/username)

## Behavior

**On Success**
- Add labels: `first-timer`, `welcome`
- Comment:
🎉 Thanks for your first contribution!
Please check CONTRIBUTING.md and CODE_OF_CONDUCT.md.

**On Failure**
- Add label: `needs-fix`
- Comment:
⚠️ Please fix your PR:
- Only edit Contributors.md
- Add one entry only
- Use correct format

## Security
- Do not run code from forks
- Use safe validation (API or internal script)

## Acceptance Criteria
- Runs on PR open/sync
- Correct labels and comments applied
- Safe for forked PRs

## Test
- Valid PR → success
- Invalid PR → failure