Publish action to GitHub Marketplace

Summary

Publish action-pinner to the GitHub Marketplace so it's discoverable as a GitHub Action (alongside the existing npm/CLI distribution).

What's needed

Must-have

1. Action-specific entrypoint -- Create src/action.ts that reads inputs via @actions/core (getInput(), setOutput(), setFailed()) and calls the library functions directly (no CLI parsing).

2. Bundle with @vercel/ncc -- GitHub Actions requires a self-contained dist/index.js. Add ncc to build pipeline to bundle the action entrypoint + all dependencies into one file. node_modules/ should NOT be committed.

3. Add @actions/core dependency -- For input/output/failure handling in CI.

4. Separate build targets -- CLI build (current tsc) and action build (ncc build src/action.ts -o dist). May need to restructure dist/ or use dist/action/ for the bundled output.

To publish

5. Create a GitHub Release and check "Publish this Action to the GitHub Marketplace" -- that's it once the above is done.

Already done

• action.yml with inputs, outputs, branding (lock icon, blue)
• runs.using: node20, main: dist/index.js
• Repository is public
• README has usage examples

Effort estimate

~2-3 hours

Start by reading the existing TypeScript build configuration and package scripts, then inspect action.yml and the current dist/index.js target. Add the action entrypoint and separate ncc bundle while preserving the CLI build; done means the bundled dist/index.js matches action.yml, dependencies are self-contained, and the action can be released to the Marketplace.