Add contributors image generation action

[skyash-dev]

#1
This introduces a reusable github action that generates a contributors image from the .all-contributorsrc file.

What it does:

• reads contributors from .all-contributorsrc
• fetches avatar images
• renders a grid image (contributors.png)
• commits the image to a dedicated branch
• opens a pull request automatically

implementation details:

• uses github api (octokit)
• handles branch creation and force updates to keep it in sync with base
• avoids duplicate PRs by checking existing ones
• separates image generation logic

tests:

• i have tested image generation logic locally.
• also tested octokit workflow on https://github.com/skyash-dev/test-contributors-png

GITHUB_TOKEN must be configured with the appropriate workflow permissions:

• enable read and write permissions
• enable allow github actions to create and approve pull requests

[skyash-dev]

we can hardcode owner with processing?