Tags: StemSplit/stemsplit-github-action

docs: marketplace summary, listing copy, and action description

Add MARKETPLACE.md, README/action.yml blurbs, and release notes blockquote
for GitHub Marketplace short description.

feat: initial stemsplit-github-action release

Add official GitHub Action for StemSplit API v1 — create jobs from
public source URLs, optional wait/poll, and balance checks.