init: centralized agent skills for Claude Code + Codex

- skills/ shared by both tools (open Agent Skills standard)
- portable cross-skill refs (root-relative, no ~/.claude hardcode)
- bin/link.sh bootstrap for non-Nix machines
- nix/home.nix + flake.nix for home-manager

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014SK5Lo7LQfwLRVdCv1A8uF

skills/impeccable/scripts/detector/shared/page.mjs

@@ -0,0 +1,7 @@
+/** Check if content looks like a full page (not a component/partial) */
+function isFullPage(content) {
+  const stripped = content.replace(/<!--[\s\S]*?-->/g, '');
+  return /<!doctype\s|<html[\s>]|<head[\s>]/i.test(stripped);
+}
+
+export { isFullPage };