chore: update flake, agents lib, and clean up tracked dotfiles

- Remove .pi* and .td-root files from git index (now in .gitignore)
- Update flake.lock and flake.nix
- Add shells/coding.nix, remove shells/opencode.nix
- Update lib/agents.nix, lib/coding-rules.nix
- Update modules/home-manager/coding/agents/pi.nix
- Update tests for agents and coding-rules
- Update .gitignore

flake.nix

@@ -21,6 +21,12 @@
       url = "github:Fission-AI/OpenSpec";
       inputs.nixpkgs.follows = "nixpkgs";
     };
+
+    # Agent definitions and coding rules
+    agents = {
+      url = "git+https://code.m3ta.dev/m3tam3re/AGENTS";
+      flake = false;
+    };
   };
 
   outputs = {
@@ -81,11 +87,11 @@
 
     # Development shells for various programming environments
     # Usage: nix develop .#<shell-name>
-    # Available shells: default, python, devops, opencode
+    # Available shells: default, python, devops, coding
     devShells = forAllSystems (system: let
       pkgs = pkgsFor system;
     in
-      import ./shells {inherit pkgs inputs;});
+      import ./shells {inherit pkgs inputs; agents = inputs.agents;});
 
     # Formatter for 'nix fmt'
     formatter = forAllSystems (system: (pkgsFor system).alejandra);