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.

modules/capabilities/ai.nix

@@ -25,6 +25,11 @@ in
         plan_mode_reasoning_effort = "high";
         service_tier = "default";
         approvals_reviewer = "auto_review";
+        features.hooks = true;
+        notify = [
+          "${pkgs.llm-agents.agent-deck}/bin/agent-deck"
+          "codex-notify"
+        ];
 
         tui = {
           status_line = [
@@ -109,7 +114,9 @@ in
         pkgs.llm-agents.codex
         pkgs.llm-agents.pi
         pkgs.llm-agents.claude-code
+        pkgs.llm-agents.agent-deck
         pkgs.llm-agents.herdr
+        pkgs.tmux
         pkgs.ctx7
       ];
 
@@ -156,6 +163,27 @@ in
         be modified as authorized.
       '';
 
-      home.file.".codex/skills/nixos-maintainer".source = ./skills/nixos-maintainer;
+      home.file = {
+        ".agents/plugins/marketplace.json".text = builtins.toJSON {
+          name = "nix-managed";
+          plugins = [
+            {
+              name = "ponytail";
+              source = {
+                source = "local";
+                path = "./.codex/plugins/ponytail";
+              };
+              policy = {
+                installation = "AVAILABLE";
+                authentication = "ON_INSTALL";
+              };
+              category = "Productivity";
+            }
+          ];
+        };
+        ".agents/skills/context7-mcp".source = ./skills/context7-mcp;
+        ".agents/skills/nixos-maintainer".source = ./skills/nixos-maintainer;
+        ".codex/plugins/ponytail".source = inputs.ponytail;
+      };
     };
 }