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

nix/home.nix

@@ -0,0 +1,17 @@
+# Home-manager module: link centralized agent skills into Claude Code + Codex.
+# Usage: add this flake as an input, then import this module in your home config.
+#
+#   inputs.agent-skills.url = "git+https://git.naps.pt/naps62/agent-skills.git";
+#   # in home.nix imports: inputs.agent-skills.homeModules.default
+#
+# recursive=true links each FILE individually, so machine-local skills can still
+# live alongside the managed ones in the same dir (a whole-dir symlink would not).
+{ agent-skills }:
+{ ... }:
+{
+  home.file = {
+    ".claude/skills" = { source = "${agent-skills}/skills"; recursive = true; };
+    ".agents/skills" = { source = "${agent-skills}/skills"; recursive = true; };
+    ".claude/commands" = { source = "${agent-skills}/commands"; recursive = true; };
+  };
+}