feat: add agent session tooling and shared skills

Install agent-deck and tmux, wire Codex notifications and hooks, and expose shared Context7/NixOS skills plus the Nix-managed Ponytail plugin marketplace.

Preserve the accumulated input updates, including the older Elephant revision; its rollback rationale was not recorded. Flake evaluation passed; no activation or agent-session pilot performed during cleanup.

flake.nix

@@ -29,6 +29,10 @@
       url = "github:noctalia-dev/noctalia";
       inputs.nixpkgs.follows = "nixpkgs";
     };
+    ponytail = {
+      url = "github:DietrichGebert/ponytail";
+      flake = false;
+    };
     sops-nix = {
       url = "github:Mic92/sops-nix";
       inputs.nixpkgs.follows = "nixpkgs";