Publish runx-action to GitHub Actions Marketplace

[auscaster]

Goal\n\nPublish the existing v0.1.0 release to GitHub Actions Marketplace.\n\n## Current state\n\n- Public repo exists: https://github.com/runxhq/runx-action\n- Root action.yml exists\n- Smoke workflow passed\n- Release exists: https://github.com/runxhq/runx-action/releases/tag/v0.1.0\n- Major tag exists: v0\n\n## Manual/browser step\n\nGitHub Marketplace publishing requires editing/drafting the release in the GitHub UI and selecting "Publish this Action to the GitHub Marketplace". If the checkbox is disabled, accept the GitHub Marketplace Developer Agreement for the runxhq org first.\n\nSuggested categories:\n\n- Primary: Utilities\n- Secondary: Continuous integration\n\n## Guardrails\n\nDo not claim n8n/Zapier/Make/Pipedream cloud support from this action. This is a GitHub Actions runner integration that executes local runx skill through the CLI. Cloud orchestrator listings still require the hosted run-skill API.