fix(skills): report YAML frontmatter parse errors and serialize SKILL.md safely - #1321

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/cache/restore@0057852bfaa89a56745cba8c7296529d2fc39830, actions/cache/save@0057852bfaa89a56745cba8c7296529d2fc39830, actions/cache@0057852bfaa89a56745cba8c7296529d2fc39830, actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5, actions/setup-node@49933ea5288caeca8642d1e84afbd3f7d6820020, pnpm/action-setup@b906affcce14559ad1aafd4ab0e942779e9f58b1. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/