docs: add provenance comments — what was replaced, why, what was removed

Each action.yml now documents:
- Original upstream repo (GitHub Actions)
- Why it was rewritten for self-hosted Gitea
- What was removed from the original
- Author and date

setup-node/action.yml

@@ -1,3 +1,13 @@
+# Tinqs Setup Node — replaces actions/setup-node@v4
+# Original: https://github.com/actions/setup-node (GitHub-hosted, Node.js action)
+# Why rewritten: Original depends on GitHub tool cache and @actions/core Node.js package.
+#   Self-hosted Gitea runners don't have the tool cache infrastructure.
+#   This composite action installs Node via NodeSource APT repo and pnpm via npm.
+#   Skips install if correct major version already present (pre-baked runner image).
+# Removed from original: tool cache, .nvmrc/.node-version file, registry-url, cache (yarn/pnpm).
+# Author: Ozan / Claude Code — 2026-05-22
+# Part of: tinqs/ci (Tinqs CI toolchain)
+
 name: 'Tinqs Setup Node'
 description: 'Install Node.js and pnpm (replaces actions/setup-node)'