Add GitHub Actions card generation

[liuchong]

Summary

• Add first-party GitHub Actions support for generating static streak stats SVGs directly from this repository.
• Reuse the existing PHP stats, cache, and card rendering logic through a small shared generator function and CLI entrypoint.
• Document how profile repositories can generate and commit profile/streak.svg without depending on the public hosted endpoint.

Test plan

• Ran npx prettier --check README.md action.yml src/index.php src/generator.php bin/generate-card.php.
• Verified the action in a profile repository with workflow_dispatch; it generated and committed profile/streak.svg.
• Confirmed the generated card can be embedded from README via ![GitHub Streak](./profile/streak.svg).

Notes

• The workflow example uses GITHUB_TOKEN by default.
• For private contribution data, users should store a PAT in GitHub Secrets and reference it from the token input.

[Copilot]

Pull request overview

Adds first-party GitHub Actions support for generating static streak stats SVGs using the existing PHP stats/card pipeline.

Changes:

• Extracts shared streak-stat generation into src/generator.php and uses it from src/index.php.
• Adds a CLI entrypoint and composite action.yml for generating SVG output in workflows.
• Documents profile README workflow usage in README.md.

Reviewed changes

Copilot reviewed 4 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/index.php	Reuses the shared generator for HTTP requests.
src/generator.php	Adds reusable stats generation with cache support.
bin/generate-card.php	Adds CLI/action entrypoint for parsing options and writing SVG output.
action.yml	Defines the composite GitHub Action.
README.md	Documents workflow setup and embedding generated SVGs.

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[DenverCoder1]

I love this approach! Thanks so much for the contribution!

This should reduce a lot of the server error complaints since it does not rely on Heroku's availability.

[liuchong]

All review feedback has been addressed. The workflow trigger suggestion was added, and the README now includes a dedicated section showing how to embed the generated static SVG in a profile README.
Please check again 👀

[DenverCoder1]

Thank you for contributing!

Next we should update the demo site to support generating a customized workflow file, but that can be saved for another PR